posted by John .
1. It's your problem, not my problem.
2. It's your problem, and it's not my problem.
3. It's your problem, but it's not my problem.
(What does #1 mean? Does #1 mean #2 or #3? Or do you have another expression which is the full form of #1?)
4. I think the girl (to be) her.
5. I think the girl (to be) she.
(Which one is correct?)
#1 means that "you" need to take care of the problem, not "me." In other words, whoever the speaker is doesn't feel responsible for whatever the other person did!
2 and 3 are grammatically OK, but not as smooth as 1.
5 is correct -- correct phrasing is "I think the girl is she." (Out of context, it's hard to tell what it means.) You use "she" rather than "her" because any form of the verb to be, when used as a main verb of a clause, requires a pronoun after it to be in the same case as a subject. Think of the verb "to be" as an equals mark in math. Here are some examples:
My brother is a retired mechanic.
My brother = a retired mechanic.
The cat is a ginger tabby.
The cat = a ginger tabby.
(Is Mrs. Smith there?) This is she.
(Is Mrs. Smith there?) This = she.