Summary

Defining and non-defining relative clauses:

Defining relative clauses
Identify, define (No commas)

Pronouns:
Who -- people
Which -- things animals
That -- people and things

If THAT is followed by a noun or pronoun, it can be omitted.
NEVER omit the pronoun if it is followed by a verb!

Non-defining relative clauses
Give extra information (Between commas)

Pronouns:
Who -- people
Which -- things animals

The pronouns can NEVER be omitted