A correspondent writes from Poland about the use of was/were in conditional sentences. He was taught to say If I were, etc., and was advised that If I was, etc. was substandard. But he's noticed that grammars today seem to be saying the opposite. Some grammars say that was forms are normal and were forms are formal. Some say that the two forms are interchangeable. 'Are we in a sort of transitional period?', he asks. Maybe, he adds, were is used only in stock phrases, such as If I were you.
Yes, we have to get that one out of the way, for a start. There are a couple of expressions where the were is idiomatic - as it were is the clearest case, as it allows no substitution of was. Similarly, when the conditional is inverted, was isn't possible: I would like to go, were I not working. If I were you is slightly less fixed, as it's possible to say if I was you as an informal variant. This has become increasingly common in standard English over recent years, and it's the norm in many regional dialects. It isn't that if I were you is formal, though, in standard English; it's stylistically neutral.
This differs from the situation in the 3rd person singular, where the was form seems to be the neutral one these days, with were becoming more formal. But it's a grey area. I heard a discussion the other day which went something like this:
A If Jane was right for the part, I'd cast her.
B But that's the point. Is she right?
A Well if she were, I'd cast her, that's all I'm saying...
The stress fell heavily on were. If it weren't for the extra emphasis, one might say that here the two words were interchangeable. I suspect that A could just as easily have said If Jane were right.... On the other hand, I doubt whether A would have said, with emphasis, Well, if she was .... Phonological highlighting of the contrast seems to make a difference.
I'm not sure about this being a transitional period, though maybe the pace of change is hotting up. Usage issues of this kind were being discussed in Fowler, nearly a century ago. And indeed, you can trace uncertainty between was and were back several hundred years. What I think has happened is that the attitude of grammarians has changed. Formally they would give credence only to the formal options. Today they recognize that everyday usage includes both. A full discussion would need to recognize both a formal/informal contrast and a speech/writing contrast. Personally I (think I) use was as my norm in speech, reserving were for more formal contexts. In writing, I (think I) use were as my norm. I have to say 'think' as I don't use either all that often!
There's an excellent discussion of the various possibilities in The Cambridge Grammar of the English Language, pp. 86-8.