A restrictive clause is doubly expressive;—it says one thing directly and another by implication. In the sentence quoted the clause implies that there are people who do change their opinions.
Usually the restrictive clause is near the word it modifies, is not set off by a comma, and is introduced by the relative pronoun that, or a conjunctive adverb, or such a phrase as in which, by which, etc. No adjective clause, however, should be tested by any or even all of these accidents, but rather solely by the purpose for which the author used it.
2. The unrestrictive adjective clause.—This is one that merely adds a thought to some idea already expressed. The additional thought is often valuable, but it is never necessary.—“The window of the little parlor looked down upon the water, which had made friends with its painted ceiling, and bestowed tremulous golden smiles upon it when the sun shone.”—Howells. The clause here is evidently not used to point out which water is meant, but to tell something further about the water, something unnecessary to the truth of the sentence, but valuable in giving the reader a beautiful picture. Such a clause is sometimes merely a definition or expansion of a term already used; for example, “Style, which is the peculiar manner in which a writer expresses his thoughts, depends to some extent upon the age in which he lives.”
Introductory Word.—The adjective clause, being a dependent proposition and modifying a substantive, needs some word to indicate its subordination and join it to its substantive. This word may be several parts of speech.
1. A relative pronoun,—who, which, that. This pronoun, besides serving as a connective, is a necessary element of the clause. It may be—
(a) Subject of the verb,—“The nature of his subject compelled him to use many words that would have made Quintilian stare and gasp.”—Macaulay.
(b) Direct object of a verb,—“So I had arrived in Venice, and I had felt the influence of that complex spell which she lays upon the stranger.”—Howells.
(c) Indirect object of a verb,—“We must be as courteous to a man as we are to a picture, which we are willing to give the advantage of a good light.”—Emerson.
(d) Object of a preposition,—“Other men are lenses through which we read our own minds.”—Emerson.
(e) Subjective complement,—“He will never be the hero that his brother was.”