In the sentence "It is she", which is the subject?

If she is the subject, what is then the function of it? If it is the subject, then shouldn't the sentence be It is her since she is a subject pronoun? Thanks!

This is an interesting question. In the Original Poster's sentence she is indeed the nominative case pronoun. It is also true that we associate this case marking with the subjects of finite verbs - such as the verb is in the original example.

However, occasionally we find nominative case pronouns in non-subject positions. Here she is in fact not the subject but the complement of the verb BE. In normal usage we would expect to see accusative case her here:

• It is her.

This usage of the nominative case is unusual, but it is recognised and described in grammars such as The Cambridge Grammar of the English Language, (Huddleston & Pullum 2002 p.459). As they point out, many speakers would find this usage rather 'pedantic'. Doubtless some people in the comments here are going to say it is a form of hyper-correction (I don't entirely disagree). This usage is definitely extremely formal, and notice it is even less acceptable if we couple it with an informal contraction of the verb:

• It's she!

However, returning to the Original Poster's question, what then is the subject in this sentence? She here is nominative only in terms the name given to the pronoun used. It is not in subject position and is the complement of the verb BE. The subject then, by a process of elimination, must surely be it?

However, we do not have to take this for granted, there are several tests we can do:

Firstly, subjects normally occupy the position before the verb. This is obviously the case here.

Secondly, subjects invert with the auxiliary verb to form questions:

• Is it she?

Here we see it inverted with is. Again, this seems to show that it is the subject. Thirdly, we use the subject and auxiliary to form question tags:

• It is she, is it not?

Again here it can be shown to be the subject.

Fourthly in terms of case, although in standard English we may occasionally find nominative pronouns in non-subject positions, we hardly find accusative ones in subject position, unless co-ordinated. If we substitute it with a masculine pronoun, for example we would get:

• He is she.

This is marginally acceptable. However...

• Him is she.

... definitely isn't! Again this shows the first pronoun to be the subject.

Lastly every sentence in English, ignoring imperatives for the present purposes, is comprised of a subject and a predicate - where the predicate must include the verb. If we want to reiterate the sentence, we would probably get the following:

• It is she! It is!

In the second sentence here, is is taking code. This means that it is standing in for the rest of the Verb Phrase. In this instance, is stands for: is she. This shows that she is in the Verb Phrase, and therefore in the predicate. Also, the fact that there are only two words in the new sentence shows that, because the predicate is is, it must be the subject.

What is the subject of It is she? It is it!

In classical grammar, it is the subject and she the subject predicate, which is in the same (nominative) case as the subject.

Your assumption that she is a ''subject pronoun'' is wrong. It is a nominative pronoun, and the nominative is not used only in the subject position but also in subject predicates.