All of the statements you provided about relative clauses are correct:
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A relative clause is a type of subordinate clause. - It cannot stand alone as a complete sentence and is dependent on the main clause.
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A relative clause begins with a relative pronoun. - Common relative pronouns include "who," "whom," "whose," "which," and "that."
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A relative clause is also known as an adjective clause. - This is because it often modifies a noun, just like an adjective does.
Therefore, each statement accurately describes characteristics of relative clauses.