Defining relative clauses are composed of a relative pronoun (sometimes OMITTED), a verb, and OPTIONAL other elements such as the SUBJECT or object of the verb. Commas are not used to separate defining relative clauses from the rest of the SENTENCE. Commas or parentheses are used to separate non-defining relative clauses from the rest of the sentence.
Defining relative clauses are composed of a relative pronoun (sometimes omitted), a verb, and optional other elements such as the subject or object of the verb. Commas are not used to separate defining relative clauses from the rest of the sentence. Commas or parentheses are used to separate non-defining relative clauses from the rest of the sentence.
Non-defining relative clauses give us more non-essential information about a noun. They are also known as non-restrictive relative clauses and they are particularly common in written English. We must USE commas when writing them to separate them from the rest of the sentence. ... The relativepronoun cannot be 
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