I didn't say "between you and I" is grammatically incorrect. But I'm wondering with what standard you are saying "grammatically correct"? I am just curious to know it.
Only 'between you and me' is grammatically correct.
'Between you and I' is a very common mistake among native English speakers. You will find that some native speakers even insist that 'between you and I' is better than 'between you and me'.
There are 3 ways to demonstrate that this is a mistake:
Excellent explanation MrP. Even though I'm not a native speaker, and hence I have no authority whatsoever, it is clear that, as you say, one can't use "I", because in the example one has a preposition.
JTT: Knowledgeable language sources defend it because it is a natural part of language. Grammar isn't prescription foisted upon the users of language. Some people use this form in their speech, some don't. Some use double modals, some don't.
This doesn't make it wrong as Mr P suggests. Here, below we see that Shakespeare used "between you and I". What you have to keep in mind is
Sounds like this fictitious character, ole Humpty, and the users of the English language have a much better handle on language than the prescriptivists, eh, Abbie?