U. 29 -Relative clauses & relative pronouns
Who = persons, pets (name); restrictive and non-restrictive
Whom = used after preposition (for whom, by whom, to whom etc.)
Whose = possessive (restrictive and non-restrictrive; persons and things)
Which = things, referring to main clause
That = persons (informal), things
Which = non-restrictive clause (extra infomation, not necessary to understand the meaning) -> These diamond rings, which are beautiful, belong to the queen.
That = restrictive clause (essential information, necessary to understand the meaning; no commas)-> The thing that annoys me the most is tests after Easter.
Leave out the relative pronoun (who/which/that) if it is not the subject in the sentence.