Because English speakers employing some varieties of informal usage will say,
"Me and Joe went to the ballgame"
and may be corrected to the more formal usage,
"Joe and I went to the ballgame"
many people to jump to the false conclusion that
"I" is always a more correct substitution for "me"; and, by way of parallel,
"he" for "him," "she" for "her" and "they" for "them. "
So it is that we encounter the following: (Shown with their grammatically correct alternatives):
|between you and I||between you and me|
|a gift for you and he||a gift for you and him|
|That is John and I's plan.||That is John's and my plan.|
|I met John and she earlier.||I met John and her earlier.|
Consider the functions of case indicated in the following chart
Subject of Sentence
|John, I, you, he, she, it, we, they|
Object of Verb or Preposition
|John, me, you, him, her, it, us, them|
| John's, my, your, his, her, its, our, their
John's, mine, yours, his, hers, its, ours, theirs
Even if one does not understand when to use these distinctions, a simple test helps:
break up any one of the wrong examples given above at the word "and".
For example, out of "a gift for you and he" generate two phrases
a. a gift for you, and
b. a gift for he.
Form a. is correct, since you doesn't change with case, but b. clearly needs to use him. A native English ear can tell this.
In similar manner, decompose John and I's plan into
a. John plan and
b. I's plan.
These clearly need to be John's plan, and my plan.
RESIST such neologisms as "between you and I." They are instances of inconsistent usage and signal little more than a sense of inferiority coupled with presumptuousness, as if erudition or higher social status could be achieved by merely avoiding use of the objective pronouns.
(Note: I normally do not fret over neologisms; it is like commanding the tide to stop. Besides, they tend to be faddish. However, the misuse has become common even among university faculty to the point where those of us who clearly know how to use case have to justify its usage to those benighted manglers of the English tongue who imagine they are obliged to correct us!)
See, also: Catalog of Inductive Grammar & Usage