ASK DOCTOR DOROTHY PASTENSE FULLSTOP:
I am confused. Someone told me that to say 'If I was you' is wrong and should be 'If I were you'. But isn't that grammatically incorrect? After all, I was... or were... taught that 'be' in the past tense was I was, you were, he was, she was, it was, they were, we were...
For this I do feel sorry for learners of English, particularly as many British English native speakers do it the wrong way.
But is it the wrong way?
Well, on this occasion, the opinions are definitely split.
It is all about something called the subjunctive. Yes, this is a grammar term not featured in the book because it wasn't thought important enough, but many believe it is.
Subjunctive forms are rare, particularly in spoken English. They are used when we want to talk about things that do not yet exist but certainly could do. The subjunctive is the base form - or the infinitive form - of the verb. It occurs where we would normally add -s to the third person (he, she, it) but do not. So, for example, instead of saying he pays, we say he pay.
So when is it used? They are in sentences usually made up of two clauses (remember, a clause is that part of the sentence that usually - but not always - has a verb in it) and are used when things are demanded, requested or suggested. They often follow the word that. They do not follow the rules that we assume for verbs used in the first and third persons: I suggested that he take aspirin. She demanded that he be put in prison. They often occur in adverbial clauses (a group of words which plays the role of an adverb, and will contain a subject and a verb): It doesn't matter how we do it, whether it be once in a while or every day, it has to be done. (Here, the adverbial phrase in a while or every day is used as a similar meaning to the single adverb occasionally.)
For contrast, when the clause expresses facts, this is known as the indicative (mood). He arrived on time. She pays the bills. But at the end of the day, the subjunctive is rarely used in conversation, so don't have too many kittens about it.
Simple, right? The subjunctive is now absolutely clear, true? Not when it comes to the verb 'to be', particularly when it is used in unreal conditionals.
There's a general view that with unreal conditionals, in the if clause, instead of saying If I was you should say If I were: If I were a rich man, I wouldn't have to work hard. If I were you, I'd revise your subjunctives and indicatives. Now saying or writing 'If I were' is Standard English as the speakers (or singers) are stating a situation that is, was, or may have been hypothetically, possible.
But even Standard English would call this ungrammatical: English lessons were more likely to go wrong if the teacher were allowed to do what they liked. Why? It's not a sentence that expresses an unreal or hypothetical meaning.
However, there's no need to worry too much. Standard English is also quite happy for you to say If I was a rich man, I'd give some money to charity. If I was you, I'd sort out these differences. It's just that those people who are grammar perfectionists say you shouldn't, and you'd be surprised how many of these people hold positions in high places. They would even state that it is correct to say Unless I be wrong, this solution is never going to be universally agreed.
So the advice is this: In informal situations, or if you're not sure, then you can say If I was..., I would/could/should, etc... (or if I were... I would/could/should..., and nobody would really notice) in an unreal conditional sentence. It is standard English, so it shouldn't be marked as incorrect. In formal situations, particularly if you're going to be in a situation where you're talking or writing to someone pompous, use If I were... You know, if he were as pedantic as you at grammar, I would call him a pompous idiot!