Relative clauses in English are used to provide additional information about something without starting a new sentence.

For example, if a girl is talking to our friend and we ask another friend if they know her, we can say:

A girl is talking to Tom. Do you know the girl?

If we ask only the following question, our friend won't know which girl we're talking about:

Do you know the girl …

Using the relative pronoun who, we introduce the relative clause that defines the girl:

Do you know the girl who is talking to Tom?

Relative Clauses – Compound Adverbs