like any other defining relative clause
(use link in upper right corner to go back there)
Find the subject of the relative clause
(not the main clause!)
If the relative pronoun is not the subject...
...you can simply leave it out of the sentence.
subjects of relative clauses are marked
contact construction impossible
This is the car which was here yesterday.
He met a girl who is better than her.
contact construction possible
This is the car which I saw here yesterday.
(This is the car I saw here yesterday.)
He met a girl who he likes better than her.
(He met a girl he likes better than her.)