Learn Hardcore French: L'air est humide ce soir.

Air is a masculine noun: le air
But le becomes l' in front of a word that starts with a vowel sound (a, e, i, o, u, or silent h).
So le air contracts to l'air.

You can’t say just air on its own here; you need L'air to mean The air.

How do you know that air is masculine?

You can’t usually guess gender from meaning; you have to learn it with the word:

The dictionary will show air n.m. (for nom masculin).
When you learn a noun, it’s best to memorize it with its article: l’air (masc.), la table, le soir, etc.

There’s no rule that all words ending in -r are masculine, so air just has to be memorized as masculine.

Why is it est and not something like fait as in il fait humide?

Both structures exist, but they’re used slightly differently:

L’air est humide. = The air is humid.
You’re describing the air itself.
Il fait humide. = roughly It is humid.
This is a common weather expression, not tied to a specific noun like l’air.

So:

Focusing on the air: L’air est humide.
Talking about the overall weather: Il fait humide.

Both are correct, but they’re not built the same way grammatically.

Why is humide after est? Could it go somewhere else?

With the verb être (to be), French adjectives normally come after the verb:

L’air est humide. = The air is humid.
Le temps est froid. = The weather is cold.

You cannot move humide in front of est here.
You can change the order of other parts of the sentence, but subject – être – adjective is the normal pattern:

Ce soir, l’air est humide. (still correct)
L’air est humide ce soir. (original)

But not: L’air humide est ce soir. (incorrect)

Does humide change for gender or number?

Yes, in theory, but:

Masculine singular: humide
Feminine singular: humide
Masculine plural: humides
Feminine plural: humides

So in the singular, humide looks the same for masculine and feminine.

Examples:

L’air est humide. (masc. sg.)
La salle est humide. (fem. sg.)
Les murs sont humides. (plur.)

In L’air est humide, you can’t see the agreement, but the rule is: the adjective agrees with the noun in gender and number.

What’s the difference between humide and mouillé?

Both relate to moisture, but they’re used differently:

humide = humid, damp, moist
Used for air, climate, a room, or something that is slightly wet:
- L’air est humide. (The air is humid.)
- Une cave humide. (A damp cellar.)
mouillé(e) = wet, soaked
Used for things that are properly wet:
- Mes vêtements sont mouillés. (My clothes are wet.)
- Le sol est mouillé. (The ground is wet.)

So L’air est mouillé would sound strange; humide is the natural choice.

Why is it ce soir and not cet soir or cette soir?

In this sentence, ce soir is an expression meaning this evening / tonight:

ce here is not an article; it’s part of a fixed time expression.
soir is masculine: le soir.
The demonstrative that goes with a masculine noun is ce (before a consonant sound):
- ce soir (this evening)
- ce matin (this morning)
- ce lundi (this Monday)

You would only use cet before a masculine word beginning with a vowel sound, like:

cet après-midi (this afternoon)
cet homme (this man)

And cette is for feminine nouns:

cette semaine (this week)

Why is ce soir at the end? Could it come at the beginning?

Both are correct:

L’air est humide ce soir.
Ce soir, l’air est humide.

French often puts time expressions at the beginning or at the end of the sentence.
The meaning doesn’t really change; it’s more about rhythm or emphasis.

You would not normally put ce soir in the middle like:

L’air ce soir est humide. (possible but unusual and marked)

What’s the difference between soir and nuit? When do I use ce soir vs cette nuit?

le soir = evening (roughly from late afternoon until you go to bed)
la nuit = night (when it’s really night-time / dark and people are normally asleep)

So:

ce soir = this evening / tonight (early night, still “evening” in feel)
cette nuit = tonight / last night (depending on context, more about the actual night-time period)

In L’air est humide ce soir, you’re talking about the evening hours, not the middle of the night.

Could I say L’air est humide aujourd’hui instead of ce soir?

Yes, that would be perfectly correct; it just changes the time frame:

L’air est humide ce soir. = The air is humid this evening / tonight.
L’air est humide aujourd’hui. = The air is humid today.

Aujourd’hui refers to the whole day; ce soir focuses on the evening.

Why not say L’air c’est humide ce soir?

In standard French, with a simple noun–adjective sentence, you don’t insert c’est like that.

You use:

[Subject] + être + [adjective]
→ L’air est humide.

C’est is used in other patterns, for example:

To talk in general: C’est humide ce soir. (It’s humid tonight.)
To identify something: C’est l’air de la mer. (It’s the sea air.)

But you don’t mix l’air and c’est in the same subject:
L’air c’est humide is incorrect in standard French.

Is there any liaison or special pronunciation in L’air est humide ce soir?

Pronunciation (in IPA) is roughly: [lɛʁ ɛ tymid sə swaʁ]

Key points:

L’air: sounds like [lɛʁ] (final r is pronounced in French).
est: [ɛ], and here you do make a liaison with humide:
- est humide → [ɛ.t‿ymid] (you pronounce the t of est)
humide: [ymid]; final e is pronounced, d is pronounced.
ce: [sə]
soir: [swaʁ] (final r pronounced)

So the flow is: L’air est‿humide ce soir. with a clear -t- sound linking est and humide.

In English we say The air is humid tonight. Is the use of the and l’ exactly the same here?

Yes, in this sentence they line up well:

English: The air is humid tonight.
French: L’air est humide ce soir.

Both languages use the definite article (the / l’) because you are talking about the air around us right now in a specific context, not air in general as an abstract concept.

Could I also say Ce soir, il fait humide instead of L’air est humide ce soir?

Yes, you can:

L’air est humide ce soir.
Focus on the air itself.
Ce soir, il fait humide.
A more neutral way to talk about the weather in general: It’s humid tonight.

Both are natural and correct. Il fait humide feels more like a typical weather comment, while L’air est humide explicitly mentions the air.