There are two types of relative clauses: defining and non-defining. Defining clauses provide essential information about a noun and are not set off by commas. Non-defining clauses provide extra information about a noun and are set off by commas. Relative pronouns introduce relative clauses and do not change form based on number or gender of the noun. Subject pronouns are used when the relative pronoun is followed by a verb, while object pronouns can be omitted in defining clauses. Relative adverbs like "where" can replace a relative pronoun and preposition.
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