relative clauses

Defining relative clauses carry essential information, because they identify which thing or person we are talking about, and they are used without commas.

This is the music which was used at the show. 

Have they found the prisoner who escaped last week?

In defining relative clauses we can use that instead of which or who.

This is the music that was used at the show. 

Have they found the prisoner that escaped last week?

Who/which/that, can be omitted if they are followed by subject + verb

Can you pass me the box (which/that) I keep in the top drawer?

You are not the man (who/that) I thought you were. 

Non-defining relative clauses are used between commas, and they add extra information which is not necessary to know who or what we are talking about.

This music, which I really like, was used at the show.

My sister, who I truly admire, is coming for Thanksgiving. 

In non-defining relative clauses (between commas) we cannot use that, and we cannot omit who/which.

This music, which I really like, was used at the show. 

My sister, whoI truly admire, is coming for Thanksgiving. 

Two subjects 

When who, which or that is the subject of a relative clause, we don’t use another pronoun or noun after it, because we can only have one subject (who, which or that).

I saw a man who was very tall. (NOT: I saw a man who he was very tall.)

That is the painting that was stolen from the gallery. (NOT: That is the painting that it was stolen from the gallery.)