An infinitive is a type of verbal, a category of verb forms which act as other parts of speech. Constructed from the stem verb plus “to,” infinitives can act as adverbs, adjectives or nouns; infinitive nouns can function in any way that other nouns can, including direct objects. (See Reference 1.) Distinguishing between infinitives acting as adverbs and those acting as direct objects requires analyzing their position and role in a sentence.
Adverbial infinitives usually occur near the beginning or end of the sentence; Towson University’s writing guide on verbals explains that they do not need to be near the verb they modify. (See Reference 2.) Whether they begin the sentence or end it, the meaning is essentially the same, although the punctuation varies slightly. Here is an example with the adverbial infinitive at the beginning: “To survive, we ate grubs.” The same adverbial infinitive could also end the sentence: “We ate grubs to survive.”
Direct Object Placement
A direct object is a noun, pronoun or, as in the case of infinitives, a noun phrase. Unlike adverbial infinitives, direct objects usually occur very near the verb. (See Reference 3.) An adverb which acts as a noun is called a nominal infinitive. (See Reference 2.) Each part of this compound sentence has a nominal infinitive as the direct object: “We wanted to eat coconuts, so we attempted to find some.” The infinitives “to eat” and “to find” are the direct objects of “wanted” and “attempted,” because they demonstrate what we wanted and what we attempted. “Coconuts” and “some” are, in turn, objects of the infinitives. (See Reference 4.)
If sentence placement does not clearly distinguish an adverbial infinitive from a nominal infinitive acting as a direct object, try some simple tests. If you can omit the infinitive and still preserve the basic sense of the sentence, then it is probably an adverb. “We ate grubs” is a complete sentence without the adverbial infinitive “to survive,” which provides additional information about why we ate them but is not structurally necessary. On the other hand, omitting the direct object should leave a more obvious gap in meaning. Omitting the nominal infinitive phrases from the second example sentence leaves us with “We wanted, so we attempted.” The sentence is substantially changed.
"In Order" Test
Another test, suggested by Towson University’s Online Writing Support, provides a surefire way to detect whether an infinitive acts as an adverb. Try inserting the words “in order” in front of “to”; if the sentence still makes sense, you have found an adverbial infinitive. (See Reference 2.) This sentence, for example, makes perfect sense: “We ate grubs in order to survive.” As for the compound sentence example, “We wanted in order to eat coconuts” does not make sense, nor does “we attempted in order to find some.” These infinitives are not adverbial.