Formal and Informal Speech
When I get home from work, I do not open the door and say "It's I." Like most people, I say, "It's me." "It's me" is informal, colloquial language. But in formal writing, I use "It's I." We adjust our language to our audience and purpose. In formal speaking and writing, a predicate noun is equal to the subject -- in person, number, and case. Thus, in formal language, pronouns used as predicate nouns should usually be the same as those used as subjects.
1. "Who's there?" "It's I!"
2. I wish to see Mr. Smith. Are you he?
3. "Do you know John Anson?" "Yes, that's he!"
4. See that poor fellow! I shouldn't like to be he.
5. "I asked to see your sons. Are these they?"
"Yes, these are they. Shall I tell you their names?"
6. "It's she! There she is!" cried the children eagerly.
7. Yes, it was he, -- the famous admiral.
8. I wish it hadn't been I who broke the window.
9. If that is the rich Mrs. Blank, I shouldn't like to be she.
10. "Who's there?" "It's we." "Who are you?"
11. The best grammarians in the village are we four girls.