French Grammar

The infinitive in French

Forms and usage of the infinitive

Forms of the infinitive

The infinitive is the absolute and impersonal form of a verb. French verbs have just one form of the basic infinitive, ending in -er, -ir or -re. For example:
Manger = eat or to eat
It is this form that is used to reference each verb in dictionaries. (English, by comparison, has two forms of the infinitive, the full infinitive with to, and the short infintive without to.)
There is also a secondary form of the infinitive, the past infinitive, formed with the infinitie of the verb être followed by the past participle of the main verb.
Avoir mangé to have eaten

Uses of the infinitive 

The infinitive is used as a subordinate verb (following a first verb) or as a noun.

1.1. The infinitive as a subordinate verb

This is the most common use of the infinitive. A number of common verbs in French, such as aimer, vouloir, détester, pouvoir,  souhaiter, can be followed directly with a verb complement in the infinitive, as in
J'aime écouter la musique - I like listening to music.
With certain other verbs such as demander, chercher, commencer, insister , the verbal complement is introduced by the proposition à
Il a commencé à réviser - He's started revising.
With other verbs, such as oublier or refuser,  the verbal complement is introduced by the preposition de
J'ai oublié de fermer la fenêtre. - I forgot to shut the window
There are also a few verbs, such as finir, after which the verbal complement can be introduced by one of two different prepositions, depending on the meaning.
Il a fini de lire le livre. - He's finished reading the book.
Il a fini par lire le livre. - He ended up reading the book.

1.2. The infinitive as verb in a subordinate clause

Occasionally the infinitive is used as the verb of a subordinate clause in which the subject of the subordinate clause is the object of the main clause. This structure is the equivalent, or contraction, of a relative clause.
J'ai vu l'enfant tomber à l'eau. - I saw the child fall in the water ( = J'ai vu l'enfant. L'enfant est tombé à l'eau, or J'ai vu l'enfant qui tombait à l'eau.)
Il m'a regardé faire la vaisselle. - He watched me do the washing up.

2. The infinitive used as a noun

French differs from English, insofar as the infinitive is the only form of the verb that can be used as a noun . Unlike English, French does not use the present participle as a noun form of a verb.
The infinitive is used as a noun, but not like a noun. Since it remains a verb, it cannot take an article, and cannot be qualified by an adjective. It can, on the other hand, be qualified by an adverb or followed by an object.
Voire c'est croire. - Seeing is believing.
Etre ou ne pas être. - To be or not to be.
Bien manger est bon pour la santé. - Eating well is good for one's health.
Trop boire d'alcool est mauvais pour la santé. - Drinking too much alcohol is bad for one's health.
In just a few cases, verbal infinitives have taken on the quality of nouns, and can thus be preceeded by an article, and be the subject or object of a sentence.
Le savoir faire. - Skills, or knowing how to do things.
Le franc parler. - Speaking frankly, straight talk.
Son franc parler est parfois un avantage. - His frank way of speaking is sometimes an advantage.
But for most verbs, the true noun form is different:  descendre / une descente, conduire / la conduite,  commencer / le commencement,  naître / la naissance.
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