Here was the situation. She planned (or had planned, or may have planned to get someone else) to pay her gas bill, and this happened at some point in the past. So how would we describe this particular past action?

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Past simple

Caroline paid her gas bill. She paid it yesterday.

This focuses on the past act. In the first sentence she paid the gas bill, but it's not always necessary to mention this when this was paid; the fact is, she did. But the second sentence then gives this information.

Caroline didn't pay her gas bill. She now has to pay that and a little extra on the next bill because she didn't.

Well, Caroline didn't pay her bill. Maybe she forgot, and now the gas company would like her to pay this, as well as her next bill, and with a little interest on top as a kind of penalty for not paying.

Did Caroline pay her gas bill? I don't know if she paid or not.

The speaker doesn't know if Caroline paid her bill, and so is asking someone who knows her.

Did you pay your gas bill, Caroline? There's a letter from the company with red writing on it.

The speaker is asking Caroline directly as a letter suspiciously looks like a demand for a bill that is unpaid.

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Past continuous

Caroline was paying her gas bill yesterday. She was doing it online.

The speaker is assuming Caroline was paying her bill as she was supposedly doing this activity yesterday on the computer.

Caroline wasn't paying her gas bill yesterday. She was doing something else on the computer.

The speaker is fairly sure that what Caroline was doing on the computer yesterday didn't include paying the gas bill.

Was Caroline paying her gas bill yesterday? I saw her looking at air purifiers on the company's website.

The speaker is asking for confirmation as they are not sure if Caroline, while looking at air purifiers on the company's website yesterday, was also paying the bill during the time she was on the company's website.

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Past continuous with 'going to'

Caroline was going to pay her gas bill yesterday, but her phone rang and she had to leave in a hurry.

This was a planned intention by Caroline, but as she was about to, or just start the activity of paying the bill, the phone rang. For some reason the call was serious enough for her to stop and leave quickly. We don't know if the bill was eventually paid, but it's still possible that, when she returned, she completed the job. The speaker doesn't know.

Caroline wasn't going to pay her gas bill yesterday. She had other things planned.

This doesn't mean that she didn't intend to do this; it's just that she was busy at the time. However, the speaker is explaining the fact that, contrary to what the listener understood, she did not plan to pay her bill.

Were you going to pay the gas bill yesterday? I saw the bill on the kitchen table at breakfast yesterday morning, opened up.

The speaker is enquiring whether Caroline paid the bill because of the potential evidence of this activity they saw on the table yesterday, and they have chosen to talk about this in casual conversation today. Of course, it is possible that they understood in another way that she was going to pay the gas bill yesterday, and they are now asking to be sure.

Weren't you going to pay the gas bill yesterday?

The speaker is either enquiring, or they actually want to know, whether Caroline intended to pay the bill yesterday, perhaps believing that she fully intended to do this job.

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Present perfect (Something that happened in the past but with a result or consequence now)

Caroline has paid the gas bill. She told me.

Caroline paid the gas bill at some point in the past (from the context, this is most likely to be very recently), and that the speaker is aware of this now.

The date for paying the gas bill was yesterday, but Caroline hasn't paid it yet.

The gas company was expecting their bill to be paid by the end of yesterday, but unfortunately this duty has not yet been performed as of the time of the statement. 

Has Caroline paid the gas bill? The deadline for payment was yesterday.

The gas company was expecting their bill to be paid by the end of yesterday, and the speaker is asking the listener, who is connected with Caroline, if they know if she's done the job at some point up to now.

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Present perfect continuous (A continuous action that began at some point in the past and is either still continuing or has recently been completed)

Caroline has been paying the gas bill for the last few months.

She began this regular activity - of course, the one time a month - at some point in the recent past and, to our knowledge, is still doing this duty.

I'm crying because I've been paying the gas bill. It was 500 zlotys!

Caroline was paying the gas bill a short time ago, and although the job of paying has been completed, the result now is that she's very upset about how much this last bill cost!

Caroline hasn't been paying the gas bill recently, and I've just had a call from the gas company telling me to pay now or be cut off.

Paying the bill was clearly Caroline's duty, but recently she hasn't been doing this, and now there could be consequences.

Has Caroline been paying the gas bill? I just got a call from the company telling me to pay now or be cut off.

Paying the bill was meant to be Caroline's duty, but the gas company have called claiming she hasn't paid, and the speaker is uncertain, asking the listener if they know if she has paid the bill or not.

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Past perfect (A completed act before a particular time in the past)

Caroline had paid the gas bill before she went to work yesterday.

Think of the order of events here: 1. Caroline paid the gas bill. 2. She went to work. 3. The speaker is talking about this. The starting point of the story is when Caroline went to work at a certain time yesterday, but before she did so, she paid the gas bill; the first event goes into the past perfect as it happened before the second. The speaker is talking about what had happened before she went to work in the past, so this first event goes into the past perfect. The speaker seems certain that she did pay the gas bill before she went to work.

Caroline hadn't paid the gas bill before she went to work yesterday.

Caroline went to work at a certain time yesterday, but before she did so, she did not pay the gas bill when perhaps she was expected to do so.

Had Caroline paid the gas bill before she went to work yesterday?

The speaker is not sure if Caroline paid the gas bill yesterday. Perhaps this job needed to be done then, and it was needed to be done early before she left home, as she perhaps would not have time or wouldn't remember to do so at work.

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Past perfect continuous (A continuous activity that was completed before a particular time in the past)

Caroline had been paying the gas bill before she got interrupted by her friend on the phone yesterday, so I don't know if she paid it.

Think of the order of events here: 1. Caroline was paying the gas bill. 2. Her phone rang and she stopped (the activity of paying her gas bill) to talk to her friend. 3. The speaker is now talking about these two previous events. Here, we are reporting on whether Caroline was paying her gas bill before her friend called her. Since the call is the start of the events we are reporting (that is, Caroline paying her gas bill), then this goes into the past perfect continuous. The speaker does not know what happened after Caroline spoke to her friend on the phone, so we don't know for sure if, after she finished talking to her friend, she returned to the activity of paying her gas bill. 

Caroline hadn't been paying the gas bill when I came into the kitchen; she was playing Fortnite!

Following the same rules as above as how events are reported, the speaker seems certain that, at the time they walked into the kitchen in the past, Caroline was not in the process of paying the gas bill as she was expected to, but instead was playing a well-known online game on her computer!

Is it true that Caroline had been paying the gas bill by going to the post office before she switched to using the computer?

The speaker is perhaps expressing their surprise that, before paying her gas bills online, she went to the post office instead.

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Present perfect/present perfect continuous with would/could/should have

Caroline would have paid the gas bill yesterday, but she didn't have enough money in her account.

Caroline had intended to pay the gas bill at some point in the past, but she discovered that - also at some point in the past - she realised she did not have enough money to pay the bill.

Caroline could have paid the gas bill yesterday, but she decided to play Fortnite on her computer instead.

Caroline was, at some point yesterday, supposed to pay the gas bill, a duty that was important and she had the time and resources to do this, but instead she chose to play online games.

Caroline would have been paying the gas bill yesterday if you hadn't called her and told her to come to you immediately.

The speaker believes that Caroline was going to pay the gas bill yesterday - she was perhaps already sitting at the computer and about to do this, but then the listener made their phone call, instructed Caroline to see them, and as a result she didn't pay the gas bill and now there may be a problem.

But Caroline could have been paying the gas bill yesterday instead of playing Fortnite on her computer.

But she didn't do this activity yesterday when she had the time and the resources to do so, but instead she decided that playing Fortnite was a better idea, and now there's a problem.

Caroline should have been paying the gas bill yesterday. What makes you think she wasn't?

The speaker knows that paying the gas bill was a necessary duty for Caroline yesterday, but the listener seems certain she didn't. This doesn't mean that the speaker doesn't believe the listener, but they certainly expected her to do so.

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Past passive

The gas bill was paid yesterday.

Someone paid Caroline's bill yesterday. As to who paid the bill, we are not sure; it could be the speaker, or it could even have been Caroline; the parties (speaker and listener) are only interested in the bill, not who paid it. But if they need to know who paid it (assuming it's Caroline), then they would add 'by Caroline'.

The gas bill wasn't paid yesterday.

The bill was expected to be paid yesterday at some point, but it wasn't. As with passives, the question of who was to do this is perhaps not important or we don't know; our focus is only on the bill.

Was the gas bill paid yesterday?

The speaker is asking if someone (maybe Caroline?) paid the bill yesterday - if it was done at all - as they are not certain of this.

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Past passive continuous

The gas bill was being paid when you rang Caroline.

The listener rang Caroline. At the time he did so, Caroline was - perhaps - paying the bill. As with passives, it is not necessary to know who was paying the bill, and while the text suggests it was Caroline, it is always possible that her mother was doing this at the time of the call. But the focus here is on the bill, not on who was paying it.

The gas bill wasn't being paid when you called; Caroline was checking her bank account at the time.

The speaker is sure that at the time the listener called, the process of paying the bill wasn't happening. Caroline was checking her money situation at the bank and so from this we can assume she was paying, but as previous, it doesn't have to be.

Was the gas bill being paid when you spoke to Caroline?

The speaker is asking the listener whether a gas bill was being paid at a time in the past when they spoke to Caroline. 

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Causative in the past (Past perfect)

Caroline had her gas bill paid yesterday.

This tells us that Caroline's gas bill was paid, but as to who paid it is not always specified. But it still could be Caroline.

Caroline didn't have her gas bill paid yesterday.

The job of paying Caroline's gas bill wasn't done yesterday either by her or by anyone else.

Did Caroline have her gas bill paid yesterday?

The asker is perhaps uncertain if the bill was paid by Caroline or by anyone else yesterday; maybe something has happened and the asker needs to check as it's possible the non-payment of the gas bill is why something happened.

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Causative in the past (Past perfect continuous)

Caroline was having her gas bill paid yesterday.

This suggests that somebody else was paying her gas bill.

Caroline wasn't having her gas bill paid yesterday.

This does not suggest anything negative; it's possible that, while somebody else was paying the bill, yesterday was simply not the right day to do this.

Was Caroline having her gas bill paid yesterday?

Was the process of paying the gas bill yesterday being done by someone?

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Present perfect/perfect continuous with would/could/should have

Caroline would have paid the gas bill yesterday, but she didn't have enough money in her account.

I think the sentence pretty much explains itself.

Caroline could have paid the gas bill yesterday, but she decided to play Fortnite on her computer instead.

So she didn't pay the gas bill, but the possibility was there.

Caroline should have paid the gas bill yesterday. It was the last day to get the bill paid before she gets penalties.

We don't know if she paid or not, but for sure yesterday was the last day to do this.

Caroline would have been paying the gas bill yesterday if you hadn't called her and told her to come to you immediately.

Caroline was trying to find some time to pay the gas bill yesterday, but unfortunately the listener called her and any process of paying the bill that might have begun at that time, didn't happen.

Caroline could have been paying the gas bill yesterday instead of playing Fortnite on her computer.

Caroline had the opportunity to pay the bill, but she didn't because she probably got distracted and chose to play computer games instead.

Caroline should have been paying the gas bill yesterday. What makes you think she wasn't?

The speaker was reliably informed, or was certain Caroline was going to pay, but the listener isn't convinced she did this.

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Must in the past

You mean Caroline hasn't paid the gas bill? She must have done! I saw her on the company's website yesterday.

The speaker is almost certain that she paid the gas bill yesterday, even though some uncertainty of this has been suggested by the listener.

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Reported speech

Caroline told me that she paid the gas bill yesterday.

In Caroline's words yesterday. "I've paid the gas bill!" and the speaker is reporting this to the listener. Caroline stated at the time that she had just paid the bill. The speaker is then reporting what she said, but as a completed action. Note: It can also be reported that she had paid the gas bill yesterday.

Caroline didn't tell me that she paid the gas bill yesterday.

The speaker, who knows Caroline, didn't know she paid the gas bill yesterday as she didn't tell them, but something has happened that indicates she did.

Did Caroline tell you that she paid the gas bill yesterday?

The speaker is asking the listener, who perhaps knows Caroline, if she told him that she paid the bill.

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Conditionals

If Caroline had paid the bill online yesterday, we wouldn't have had to go to the gas office to do it in person.

Caroline did not pay the bill online yesterday and perhaps missed some kind of deadline, and as a result she and the speaker had to go to the gas office to pay and explain why they were late. (Third conditional)

If Caroline had paid the gas bill yesterday, I would know about this now.

The speaker is sure she didn't pay yesterday because they do not have some kind of proof about this at the time of speaking. (Mixed conditional)

If Caroline had paid the gas bill, she wouldn't be planning to pay it later today.

But she didn't pay the gas bill yesterday, and now she has to make some time later today to do this duty. (Mixed conditional)

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Historic Present

Watch this video. You see, Caroline goes to her kitchen, she sits down, opens the laptop, and - there - you see? she's paying the gas bill!

This tense is often used as a storytelling technique, usually when the speaker is commenting on events that happened in the past but is talking about them as if they were happening now. This technique is often used by sports commentators when they look at action replays.

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