That, which, who, whom and whose are considered relative pronouns when they are used to introduce a relative clause.
That's the man who was rude to my mother.
Can anyone tell me whose coat this is?
She turned off the news, which was depressing.
If a relative pronoun in a defining relative clause refers to the object of the main clause, it can be omitted:
John's the bloke (that/who) she used to be married to.
It was the squirrel (that/which) they had seen on the roof.
This is the dress (that/which) I bought yesterday.
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