complement

A small number of verbs are followed not by an object but by a complement. The verbs, sometimes called linking verbs, include:

to be to seem to appear to to become

In sentences containing these verbs, the subject and the complement refer to the same person or thing:

⦿ My uncle became a Roman Catholic.

My uncle = subject

became = verb

a Roman Catholic = complement

The complement of a sentence can be a noun, preposition, or noun phrase (just like the object), but it can also be an adjective or adjective phrase:

⦿ My uncle is very happy.

Grammatically the complement should be the same case as the subject; hence the problem people have with:

⦿ Who’s there? It is I.

Technically this is correct and, in formal written English, this is what you should use. But if you use ‘It is I’, rather than ‘It’s me’ in everyday conversation people will probably think you are being a bit finicky.

bbb

xx