Relative Clauses

I met a woman. She can speak six languages.
She -->who
I met a woman who can speak six languages.

Jim was wearing a hat. It was too big for him.
It --> that or which
Jim was wearing a hat which was too big for him.

Who… is for people (not things):
A thief is a person who steals things.
Do you know anybody who can play the piano.
I know somebody who knows you.
The people who work in the office are very friendly.

That… is for things (and sometimes people):
An aeroplane is a machine that flies.
Jack lives in a house that is 500 years old.
That is also possible for people (Do you know anybody that can play piano?), but it is usually netter to say who.

Which… is only for things:
An aeroplane is a machine which flies.
Jack lives in a house which is 500 years old.
Don’t use which for people