Defining: if we remove the relative clause the sentence doesn't usually make sense.
We don't use any commas
This is the jumper I've bought.
We can omit the relative pronoun if it's not the subject:
I have eaten all the biscuits (that) you bought.
When, where and why can be omitted:
I remember the day (when) we went skiing.
Non-defining: it gives us more information about someone or something. If we remove the relative clause it still makes sense.
We cannot use that, why or what.
We must use a comma or commas.
Mrs Evans, who is a pianist, is coming tonight.
We cannot omit where and when.
Farnborough, where I was born, is in southeast England.
Who- people or animals (to give them a personality)
Which- things or animals
When- to refer to time
Where- to refer to a place or situation
Why- to give a reason
Whose- the possessive of who and which
What- the thing(s) which. Only in defining clauses.
Whom- formal for who (basically used in writing)
That- who, which, when, where, why. Only in defining clauses.
There are a lot of people who don't like bitter things.
This is the cardigan which I got for Christmas.
Can you remember the days when you had to walk to school?
Alaska is a place where it never gets dark from May to July.
And that's the reason why I wanted to go in bed early.
There are children in the school whose parents are from abroad.
I'll give you what I promised.
That's the lawyer with whom you have to talk.
Charlie is the neighbour that I see the least.