In English grammar, the infinitive phrase is a sentence fragment typically starting with the word “to,” immediately followed by a verb, and then any other additional words to complement the verb. The phrase itself is not, however, a verb. It is very commonly used as a noun, but it can also be used as a descriptive modifier for other words. Most other languages of the world employ this versatile linguistic technique. Many of them change the form of the verb slightly with the addition of a suffix to help clearly define it as an infinitive.
The infinitive phrase is frequently referred to as the dictionary form. In a dictionary, the definition of the word “whisper” might read, “to talk quietly.” The first word of this phrase, not to be confused with the preposition “to” which denotes a relational direction, is called an article. “Talk” is normally a verb. In combination, “to talk” is the infinitive, and the additional adverb “quietly” to describe how completes the full phrase.
Infrequently, the term infinitive is also referred as a non-finite verbal form, of which there are three types in English. One, called a participle, such as in “a whispering wind,” is always a descriptive adjective. Another type, called a gerund, is always a noun, such as a library sign, “Talking is not allowed.” When a true infinitive is used as a noun, it is usually interchangeable with a gerund, as in “To talk is not allowed.” One distinctive difference of the infinitive is that it can be used as either noun, adjective or adverb.
Some languages have infinitives which do not normally combine with an article. It is said to be a bare infinitive. Although this is uncommon in English, it is the norm in most Romance, Germanic and Slavic languages. Omitting the article, the form of the verb instead is changed with the attachment of a suffix, such as -en in German and Dutch. As is more common in English, both languages do sometimes employ an article to create a full infinitive, also called a to-infinitive.
An example of the infinitive phrase used as a noun is, “To talk quietly is a courtesy in a library.” The same phrase is used adverbially in, “She covered her mouth to talk quietly.” It is sometimes used as an adjective to describe nouns and pronouns, such as, “Rather than argue, she is one to talk quietly.”
There is more to the versatility of the infinitive phrase and other non-finite verbal phrases. Despite the fact that they are no longer being used as verbs, they retain most of its characteristics and can accept additional words that verbs normally would. “She quietly asked the librarian... to harshly teach the loud and disruptive boy a lesson in good manners.” The infinitive “to teach” in this example is complemented by, not only an adverb, but also both direct and indirect object nouns. Incidentally, this infinitive has been “split” by its adverb, and there is some controversy whether this practice is stylistically appropriate English.