Defining Relative Clauses

Relative pronouns that can be used in defining relative clauses are: who (that), which (that), whose, where, when. What cannot be used. Which and that are often interchangeable. In defining clauses, however, that is commonly used instead of which after superlatives, every(thing), all, only, some(thing), any(thing), no(thing), none, little, few and much.

	Commas are not used for separating a defining relative clause from the main clause.
	Relative pronouns can be left out when they are not the subject of a defining relative clause.
	Whose can never be left out.
	Where and when are usually not left out.

That cannot be used to introduce a non-defining clause, so it cannot be used when a comma separates the relative clause from the main clause.

Which is used with a preposition/particle in:

	Formal English – this is the page in which you need to correct most of the sentences.
	Informal English – this is the page (which) you need to correct most of the sentences in.