In French grammar, an auxiliary (un auxiliaire in French) is a verb that you can combine to another (main) verb to add something (grammatical traits) to its meaning including the person, tense, aspect, voice, mode, etc.)

J'ai mangé une banane. --> I ate/have eaten a banana.

Elle est arrivée. --> She has arrived.

L'auxiliaire  precedes the verb in French. It helps conjugate it.

In French, there are two basic auxiliaries: être and avoir.

With an auxiliary, you can make compound tenses such as the passé composé, past conditional, etc. You can also set the voice (passive) of a sentence.

Oftentimes, verbs other than avoir and être can can join other verbs in the infinitive form and give them additional temporal, aspectual or modal meanings.  We call such verbs a semi-auxiliary or modal verb (un semi-auxiliaire) --> aller / faire / devoir / pouvoir, etc.

Je vais partir bientôt.. --> I'm going to leave soon

Tu dois t'assoir correctement. --> You must sit correctly 

N.B. Semi-auxiliaries are conjugated while the main verbs stay in the infinitive.

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Find below a number of sentence that include auxiliary verbs and semi-auxiliaries.

1. Chris est passé ici le mois dernier. / Chris stopped by last month.
2. Nous voulons aller au cinéma. / We want to go to the movie theater.
3. Elle va jouer au foot. / She is going to play soccer.
4. Les enfants ont dormi chez leurs amis hier. /The children slept at their friends' yesterday. 
5. Je ne peux pas soulever cette pierre. /I can't lift this stone. 

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