I did a test in a session Test Your Grammar, in Word Power Made Easy by Lewis.
1) No one but (she, her) ever made a perfect score on the test
2) I can't remember (who, whom) it was.
3) Was it (she, her) you were talking about?
4) "It is I who (is, am) the only friend you've got", she told him pointedly.
The answers given by the book are 1) her, 2) who, 3) she, 4) am.
Of course I gave the other answers. But I don't understand why I am wrong. For example, in 1), we should choose a pronoun as a subject, not object. So I think 'she' is right; in 3), instead, we should choose a object, since the sentence means "was it that you were talking about her". So I think 'her' is right here.
What do you think?