Relative pronouns: non-defining relative clauses

  These snacks, which are so easy to make, are absolutely delicious.
 

We sometimes use a relative clause in order to add a little extra, unnecessary information.

In these kinds of sentences, we use a comma before and after the clause:

My manager, who prefers bigger teams, wants to recruit more staff.

Our most recent project, which took two years to complete, was very successful.

If we remove the relative clause, the meaning of the sentence remains clear - we know which manager and project we are talking about:

            My manager wants to recruit more staff.

            Our most recent project was very successful.

So, there is a difference between these two sentences:

1.     My sister, who lives in Paris, is an architect. 

2.     My sister who lives in Paris is an architect.

In sentence number 1. we know which sister we are talking about - maybe there is only one sister or it is already clear which one we are referring to. We could just say:

My sister is an architect.

In sentence number 2. the speaker has more than one sister and uses a defining relative clause to tell us which one is an architect. It is the one who lives in Paris.  

Note that

1.     in non-defining relative clauses, we cannot use that. We have to use who or which.

My sister, who lives in Paris, is an architect.

My sister, that lives in Paris, is an architect. 

My sister who/that lives in Paris is an architect. 

2.     it is never possible to omit which and who in non-defining relative clauses:

My daughters, who are both students, live at home with me.

My daughters, who are both students, live at home with me.  

The meeting, which was on Monday, wasn't very productive.

The meeting, which was on Monday, wasn't very productive.  

3.     the rules regarding who and whom are the same as for defining relative clauses :

My manager, whom/who I really like, is retiring next year.

My manager, who prefers bigger teams, wants to recruit more staff.

If there is any doubt, it is better to use who because it is much more commonly used than whom - both spoken and written.

4.     We can use where and whose in non-defining relative clauses:

I'm going to Venice, where I first met my husband, with some friends next year.

The book, whose author is Mexican, is a best-seller.

5.     We can use prepositions in non-defining relative clauses:

Our manager, for whom we have a lot of respect, always listens to our problems.  [formal]

Our manager, who we have a lot of respect for, always listens to our problems.  [conversational]