In this sentence from Henry James, “He must have been an American, to take the picturesque so prodigiously to heart,” the infinitive phrase is the equivalent of an adverbial clause of reason, telling on what ground the main assertion rests.
The infinitive may also denote condition, or manner; for example, “You must have the imagination of a poet to transfigure them,” and, “Is everything going to suit you?” In the former sentence the infinitive phrase is equivalent to the clause, if you transfigure them. In the latter it tells how things are going on. In fact, this useful little element may accompany a predicate verb to denote almost all the relations denoted by adverbial clauses.
Sometimes it is difficult to decide whether an infinitive is a complement or a modifier. Perhaps the best way to settle the point is to ascertain what the infinitive is equivalent to, or what question it answers; for example, “If you continue your observations, you will not fail to notice that the speed of its disappearance depends on the character of the day.”—Tyndall. Here the infinitive answers the question, fail in what respect? and is equivalent to the phrase, in noticing. It is well, then, to consider it an adverbial modifier.
Perhaps the infinitive is used after no other verbs oftener than after go and use, as in the sentences, “I am going to stay until you are tired of me”; “I used to wonder what this might be.” Both go and use are intransitive verbs, but the infinitives can hardly be described as their modifiers, for the predication lies more in the infinitive than in the finite verbs, am going or used. The verb am going makes the action denoted by the infinitive intentional, and used makes it customary. Such predicates as am going to stay and used to wonder should be described either as idiomatic verb-phrases, or as consisting of an intransitive verb and an infinitive, without any attempt to establish a complementary or modifying relation between the infinitive and the verb.
Note.—The verb be is sometimes used in the sense of go or come, and may then be modified by an infinitive of purpose, as in the sentence,—“The old Master was talking about a concert he had been to hear.”—Holmes.
III. The infinitive may be associated with an adjective or an adverb.
A construction often met is a predicate adjective followed by an infinitive, especially such adjectives as sure, glad, sorry, apt, anxious. The infinitives answer such questions as sure of what? anxious about what? and so limit the meaning of the adjective; for example, “We are right willing and able to work.”—Carlyle. “He was careful to acquaint himself with the country.”
Often the adjective is first modified by the adverb enough or too; thus, “This narrative is too significant to be omitted.”—Carlyle.
When the adjective is first modified by so, the infinitive is introduced by as, and takes the place of a clause of result; as, “Be so kind as to stand.” Here the infinitive is brought into the sentence by the adjective kind, but as an infinitive of result, may be said to modify the entire predicate be so kind.
The infinitive in this construction may also be associated with an adverb; as, “He worked so swiftly as to surprise us all.” We notice that it is easy to avoid the clause here by substituting the infinitive, for the subject of the clause is the same as the subject of the principal verb.