Relative clauses. Clauses with who/that/which. Part 1
A clause is a part of a sentence. A relative clause shows us which thing or person the speaker means.

The woman who lives next door…

People who live in this country…
1. We should use who in a relative clause when we are talking about people (not things):
An architect is someone who designs buildings.

Anyone who wants to apply for the job must do so by Friday.
Also you can use that for people, but don’t use which.
2. When you talk about things, you should use that or which in a relative clause.

Where is the cheese that/which was in the fridge?

I don’t like stories that/which have unhappy endings.

That is more usual than which, but sometimes you must use which.
3. What = ‘the things(s) that’.

What happened was my fault. ( = the thing that happened)

The machine that broke down is now working again.
Remember that in relative clauses we use who/that/which, not he/she/it.

She has never spoken to the man who lives next door.