Arguably, there is little more irritating in linguistic misuse than the “hypercorrection” – the most obvious example of which is the use of the subject form of personal pronouns when in fact the objective form is required (e.g. the wrong *”for you and I” instead of the correct “for you and me”).
Unusually in a discussion of linguistic use, I have even gone as far here as to use the terms “wrong” and “correct”. This is because this misuse is an exception. Most apparently “incorrect” linguistic use is in fact “non-standard” – for example, “me and him is friends” is grammatically correct in Belfast English; it is just not Standard usage. However, “for you and I” is not non-Standard, it is just wrong – there is no dialect in which it is correct.
The reason some people use it is obvious. At school we are taught that, indeed, terms such as “me and him is friends” are non-standard (well, we are actually taught they are “wrong”, but “non-standard” would be more accurate for reasons noted above). However, we are not adequately taught why.
Many people know the solution is obvious. Imagine the phrase without the “and” – is it “for I” or “for me”?
The underlying principle, in Standard English at least, is that the subject (also known as “direct”) form of the pronoun is used when it is the subject (thus I, he, she, we, they), and the objective (also known as “oblique”) form otherwise (thus me, him, her, us, them). As a passing note, the term “accusative” (derived from Latin) is unhelpful here – most English objective forms actually derive from old “datives” (this is obvious even from modern German, where for example accusative ihn is distinguished from the dative ihm as against the sole English form “him“).
A more debatable issue is the distinction between “who” and “whom”. The latter again derives from an old dative and is in principle an objective form, to be used whenever it is not the subject (e.g. “the man whom I saw”). However, English lacks an Academie Anglaise or anything similar, and thus common usage has to be taken into account. In practice, “whom” is often avoided (“the man I saw”) or just not used (?”the man who I saw”); it is starting to be used only after prepositions, and even then only when positioning after the preposition is unavoidable – thus modern usage dictates that in practice you can have the formal “To whom did you give it?” or the informal “Who did you give it to?” but never *”To who did you give it?”.
In turn, this latter paragraph gives us the “hanging preposition” (a clause ending in a preposition, “Who did you give it to?”), which some people cite as dubious usage. Churchill is alleged to have said it was something “up with which I will not put”. There is definitely no problem with a “hanging preposition” which is part of a phrasal verb – so “this is something I won’t put up with” is fine because “to put up with” is in itself a phrasal verb. However, in practice there is really no problem at all with the “hanging preposition”, which has been apparent in English for centuries and is also used contentedly in Scandinavian languages and Afrikaans (comparison with German and, arguably at least, Dutch is redundant here as their case system leads to an entirely different structure).
However, “for you and I” is the type of usage with which none of us should put up…