Relative clauses 2. Clauses with and without who/that/which.
 
You must use who/that/which when it is the subject of the relative clause.

So you cannot say ‘The woman lives next door is a doctor’ or ‘ Where is the cheese was in the fridge?’.
 
Sometimes who/that/which is the object, you can leave it out. You can say:
 
The man who I wanted to see was away on holiday.
 
who = the man (is the object)

I - the subject

Have you found the keys that you lost?
  
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1. When who/that/which is the object, you should leave it out. You can say:

The woman I wanted to see was away.

The dress Kate bought doesn’t fit her very well.
 
Remember!

the keys you lost (not the keys you lost them)

the dress Kate bought (not the dress Kate bought it)

2. Note the position of prepositions (in/to/for) in relative clauses:

Are these books you were looking for?

The woman he fell in love with left him after a month.

the books you were looking for (not the books you were looking for them)
 
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3. You cannot use what in sentences:

Everything (that) they said was true.

I gave her all the money (that) I had.

What = ‘the thing(s) that’.

Did you hear what he said? (= the things that he said)