Why does French use avoir here?

Because it treats these states as things you possess rather than qualities you have. It is not an oddity of French alone, Spanish and Italian do the same with tener hambre and avere fame, so if you have one of those languages the pattern will already feel natural.

Is avoir l’air an avoir expression too?

It works the same way and is worth adding to the list: elle a l’air fatiguée, she looks tired. Note that the adjective after it agrees with the subject in ordinary usage, even though l’air is masculine, the version agreeing with l’air is old-fashioned.

How do I say I am hot as opposed to it is hot?

J’ai chaud is about you; il fait chaud is about the weather. And c’est chaud is about the object, the plate. Three different constructions for one English adjective, and mixing them up produces sentences that are grammatical and about the wrong thing.

Grammar referenceA1

Avoir expressions

The short answer

French says j’ai faim where English says I am hungry: the state is something you have, not something you are. The same pattern covers thirst, cold, heat, fear, age, need and being right or wrong, and none of them takes an article.

The rule

A group of common French expressions uses avoir plus a bare noun where English uses be plus an adjective. J’ai faim is literally I have hunger. The noun takes no article, which is what makes these fixed expressions rather than ordinary sentences, and it is why adding un or du breaks them. Several of the group take a preposition before their object: avoir besoin de, avoir peur de, avoir envie de, and those prepositions decide which pronoun replaces the object later.

Form	Structure	Example
Physical states	`avoirbare noun`	avoir faim, soif, chaud, froid, sommeil
Feelings	`avoirbare noun`	avoir peur, honte, mal, raison, tort
Age	`avoirnumberans`	J’ai trente ans.
With de	`avoir besoin / envie / peur de`	J’ai besoin de temps.
With à	`avoir mal à`	J’ai mal à la tête.
Intensifying	`trèsnot beaucoup`	J’ai très faim.

Part 1 of 3

The core list

Physical states: avoir faim (hungry), avoir soif (thirsty), avoir chaud (hot), avoir froid (cold), avoir sommeil (sleepy). Feelings and judgements: avoir peur (afraid), avoir honte (ashamed), avoir raison (right), avoir tort (wrong).

Age belongs here too, and it is the one that surprises English speakers most: j’ai trente ans, never je suis trente. The word ans cannot be dropped either: j’ai trente is not a sentence.

Tu as froid ? Je peux fermer la fenêtre.

Are you cold? I can close the window.

Elle a dix-huit ans depuis hier.

She turned eighteen yesterday.

Tu as raison, j’aurais dû demander.

You’re right, I should have asked.

NBAvoir raison, never être raison.

Part 2 of 3

The ones that take a preposition

Avoir besoin de (to need), avoir envie de (to want, to feel like) and avoir peur de (to be afraid of) all take de before their object, whether that object is a noun or an infinitive.

Because they take de, they take en as their pronoun: j’en ai besoin, j’en ai envie. Avoir mal takes à instead, and contracts normally: j’ai mal à la tête, au dos, aux dents.

J’ai besoin de deux jours de plus.

I need two more days.

Tu as envie de sortir ce soir ?

Do you feel like going out tonight?

Il a mal au genou depuis sa chute.

His knee has hurt since he fell.

NBAvoir mal à, contracting with le to au.

Part 3 of 3

Intensifying them

Because the noun is bare, these expressions take très rather than beaucoup: j’ai très faim, j’ai très peur. That looks wrong: très normally modifies adjectives, but it is the standard form and beaucoup here is an error.

Avoir besoin is the exception and takes grand or vraiment instead: j’ai vraiment besoin de dormir. And avoir mal takes très as well: j’ai très mal.

On a très soif, il fait quarante degrés.

We’re very thirsty, it’s forty degrees.

Elle a vraiment besoin de vacances.

She really needs a holiday.

J’ai eu très peur.

I was very frightened.

Mistakes to avoid

What learners actually get wrong

Je suis faim.
J’ai faim.
These states are had, not been. English chooses be plus an adjective and French chooses avoir plus a noun, and translating the structure rather than the meaning produces a sentence no French speaker would parse as hunger.
Je suis vingt-cinq ans.
J’ai vingt-cinq ans.
Age uses avoir, and ans cannot be omitted. Both halves of the English sentence, the verb and the missing noun, have to change together.
J’ai beaucoup faim.
J’ai très faim.
The bare noun in these expressions takes très, despite très normally belonging with adjectives. Beaucoup would be logical and is simply not what French says.
J’ai besoin du temps.
J’ai besoin de temps.
Avoir besoin de takes a bare de before an unspecified quantity, the partitive collapses after de, as it does after any expression of quantity. Du temps would mean some specific time already mentioned.

Questions

Asked and answered

Why does French use avoir here?: Because it treats these states as things you possess rather than qualities you have. It is not an oddity of French alone, Spanish and Italian do the same with tener hambre and avere fame, so if you have one of those languages the pattern will already feel natural.
Is avoir l’air an avoir expression too?: It works the same way and is worth adding to the list: elle a l’air fatiguée, she looks tired. Note that the adjective after it agrees with the subject in ordinary usage, even though l’air is masculine, the version agreeing with l’air is old-fashioned.
How do I say I am hot as opposed to it is hot?: J’ai chaud is about you; il fait chaud is about the weather. And c’est chaud is about the object, the plate. Three different constructions for one English adjective, and mixing them up produces sentences that are grammatical and about the wrong thing.

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