Adjective clauses begin with relative pronouns (who, whom, whose, which, that) or relative adverbs (when, where). Relative pronouns usually serve as subjects or objects in the clauses they introduce; another word in the clause cannot serve the same function. Relative adverbs should not be repeated by other adverbs later in the clause.

The relative pronoun that is the subject of the adjective clause, so the pronoun it cannot be added as a subject.

The relative pronoun that is the object of the verb attended. The pronoun them cannot also serve as an object.

Sometimes the relative pronoun is understood but not present in the sentence. In such cases, do not add another word with the same function as the omitted pronoun.

The relative pronoun that is understood after seminars even though it is not present in the sentence.

If the clause begins with a relative adverb, do not use another adverb with the same meaning later in the clause.

The adverb there cannot repeat the relative adverb where.