Should you always say “you and I” and avoid “you and me”? Some native speakers will tell you so, but not only does no such rule exist, but using “you and I” instead of “you and me” is plain wrong in many situations.
The rule is quite simple, actually. If “you and I” is the subject of a sentence (i.e. “you and I” are the people who are doing the action), nothing needs to be changed:
The second example is not strictly speaking wrong (it is widespread to use “you and me” this way), it’s just colloquial and should be avoided in formal speech and formal writing.
If “you and I” is an object (i.e. the action is being done to it), the correct form is “you and me”:
A simple mnemonic is to say “he” or “him” instead of “you”. Would you say “she didn’t see he and I”? I don’t think so. Similarly, you probably wouldn’t say “him and me are good friends”.