1. Defining and non-defining relative clauses

2. The girl who learns here is very pretty.
Relative clauses are relative pronouns and adverbs: who, whose, which, that, where, when and why. We use them to describe or identify what was mentioned in the main clause.
The girl who learns here is very pretty.

3. PEOPLE - WHO/THAT OBJECT/ANIMALS – WHICH/THAT POSSESSION - WHOSE

4. Who, which and that can be omitted:
When they are the object of the relative clause (when there is a noun or a subject pronoun between the relative pronouna nd the verb) – simply when the whole sentence is about ’one thing’.
The suitcase (which/that) I bought is too small.

5. Who, which and that cannot be omitted:
When they are the subject of a relative clause.
We gave the porter who carried out our luggage a big tip.

6. Whose is never omitted. That can be used instead of who, which or whom BUT IT IS NEVER USED AFTER COMMAS OR PREPOSTIONS.

7. PLACE – WHERE REASON – WHY TIME - WHEN

8. Defining relative clauses
They give necessary information essential to the meaning of the main sentence. It is not put in commas and it is introduced with who, which, that, whose, where, when or the reason/why.
I met the girl (who/that) Jake loves.

9. Non-defining relative clauses.
They give extra information and is not essential to the meaning of the main sentence. It is put in commas and is introduced with who, whom, which, whose, where or when. The relative pronoun cannot be omitted.
I met Anna, who John loves.

10. A comma can change the meaning of a sentence!

11. Question time: how many sons does she have?
A. She has two sons who are in the army.
B. She has two sons, who are in the army.

12. The answer is: She has two sons who are in the army.
More than two sons
B. She has two sons, who are in the army.
Two sons

13. Another interesting example:
I’ve got another 1 today, which will not make my dad happy.
We are talking about a 1. Without a comma the relative clause would be about today.