Avoir is irregular in almost every tense and mood. It literally means "to have," but it also works as an auxiliary verb and shows up in dozens of everyday expressions — several of which English speakers get wrong at first, simply because English uses a completely different verb for the same idea.
1. Avoir = to have, to possess
The most literal meaning — this one works just like English.
| J'ai une voiture bleue. | I have a blue car. |
| As-tu un chien ? | Do you have a dog? |
| Nous avons deux enfants. | We have two children. |
2. Avoir = to be — the #1 trap for English speakers
In a whole family of common expressions, French uses avoir exactly where English uses "to be." This is probably the single most common mistake English-speaking students make when they start out.
| J'ai faim. | I am hungry. (literally: I have hunger) |
| J'ai froid. | I am cold. |
| Il a raison. | He is right. |
| J'ai 15 ans. | I am 15 years old. |
3. Avoir as an auxiliary verb
For most French verbs, avoir is also the helping verb used to talk about the past. You'll see this a lot once you start using the passé composé.
| Nous avons déjà mangé. | We've already eaten. |
| Elle a visité Nice. | She visited Nice. |
4. Conjugation — present tense
| j'ai | I have |
| tu as | you have (informal) |
| il / elle / on a | he / she / one has |
| nous avons | we have |
| vous avez | you have (formal / plural) |
| ils / elles ont | they have |