[Tdol]

Originally Posted by "Casiopea

It's an adverb.
Used elliptically.

Q: Who ate the cake?
A: Not me. (It was not me.) *popular

Q: Who ate the cake?
A: Not I. (It was not I.) *traditional grammar

According to traditional grammarians (Prescriptivist), linking verbs such as forms of 'to be' (is, are, was, were, etc.) link the subject with its complement. Complements refer back to the subject so they are considered 'nominative' (subject) in form. Which means, pronouns coming after the linking verb 'to be' should be nominative in form: I, she, he, we, they. For example,

Q: Who ate the cake?
A: Not I.

On the phone:

Pat: Hello, may I speak with Sam, please?
Sam: This is she.

But, keep in mind, those examples are based on what Prescriptivists would advise. As for Descriptivists, they'd point out that "me" is more popular these days than "I". The reason being, the pronoun comes after the verb which is a position reserved for objects, and hence speakers tend to choose "me" over "I" in that context.

Other 'not' example:

Q: Is she coming?
A: I hope not. (I hope she is not coming)

Q: Do you want it?
A: Certainly not! (I certainly do not want it.)

Cas :) That was fun! Thanx

Which do you use? I would only use 'I' in very formal conversations.