What's the difference?
I have to admit that, until I decided to write about this and then add it to this section, the differences between these words and the participle form they take had even confused me. And I am in distinguished company: even The Times newspaper gets the verb forms mixed up (Kamm, 2015, p211, Wiedenfeld & Nicolson).
The main problem is the word lay, as there are two forms of the verb but are spelt the same.
The first is with the meaning ‘to place in a more or less horizontal position’. Its present tense form is lay, and its past and past participle forms are both laid.
The second has the meaning ‘to be in a more or less horizontal position’. The present tense form is lie, and its past form is lay.. But its past participle form is lain.
I can demonstrate this further with these examples:
Lay - Present tense (infinitive): lay; Past tense: laid; Past participle: laid: Lay the wet towel on the radiator. One hour ago I laid a wet towel on the radiator and now it's dry! I have laid a lot of wet towels on the radiator, so hopefully they'll be dry soon. The continuous/progressive form: Your clothes are laying all over the floor!
Lie - Present tense (infinitive): lie; Past tense: lay; Past participle: lain: Lie on the soft carpet. It's very comfortable! Yesterday I lay on the soft carpet and it gave me no back problems. I have lain on many soft carpets in my time but this one really feels comfortable on my back! The continuous/progressive form: Stop lying in your bed and get up and do something!
Here’s another example of how they could be used:
‘I’m going to lay a towel on the beach and then lie on it to get a suntan,’ Julie explained to her friend, Daria, who was in the kitchen. An hour later, she laid her towel on the beach. Then her phone rang as she thought about laying on it; it was Daria. 'Hi Daria,' said Julie cheerfully. 'What's up?' after she started lying on it. ‘Have you got my towel?’ asked Daria. ‘Which one?’ replied Julie, ‘you don't mean the green one?’ ‘Yes!’ said Daria. ‘Oh Daria, I’m not going to lie... I'm afraid I’ve taken your towel and I’ve already laid it on the beach and lain on it!’ 'You're not lying, are you?' replied Daria nervously.
And as you may have read, there is also the verb ‘lie’ meaning ‘not tell the truth’, but this is lie/lied/lied. And to confuse things further, the continuous/progressive form of lie is lying, the same spelling as to be in your horizontal position on the floor: You're lying! You were laying your towel on the beach and lying on it and you told me you were ill and had to stay at home!
Note that if you say I lay on the floor, this means that that this is a completed action of having been on your back on the floor. The verb ‘lie’ (as in to lie on the floor, lay being the past tense) is intransitive, meaning it cannot take a direct object. You cannot say ‘I lay something on the floor yesterday’, but you say 'I lay on the sofa when I'm tired'. But I laid on the floor is not a complete sentence as the verb requires an object. You have to have laid something on the floor. Otherwise, you could get this response: What did you lay on the floor? An egg?
If you ‘lay on something’ it is a colloquialism meaning ‘to provide something’: Let’s lay on a party! We should lay on a meal for six. Also, many recipe books can ask you to lay (the cake/bread) on a rack and stand in a cool place. And of course, as demonstrated above, hens lay eggs.
And then we have I got laid last night, but if you want to know the meaning of that, I'm not going to explain it here.
Now English wouldn’t be English without a little ambiguity somewhere. From a nursery school poem: Hens lay, but little girls lie. (What exactly? Lie on the floor or not telling the truth?)