Learn Hardcore French: Ils entrent dans une petite grotte où l'air est frais et humide. - They enter a small cave where the air is cool and damp.

Saying ils entrent une grotte is wrong in standard French, because it treats grotte as a direct object, which entrer does not usually allow.
Always think: entrer dans + room, building, place, etc.

Why is it une petite grotte and not un petit grotte?

Because grotte is grammatically feminine in French.

Feminine article: une grotte
Feminine adjective: petite (with -e at the end)

So everything that agrees with grotte must be feminine singular:

une petite grotte

If it were masculine, it would be:

un petit
- masculine noun (for example: un petit village).

Why does petite come before grotte, while frais and humide come after est?

There are two different things going on:

Adjective position with nouns
Some adjectives commonly come before the noun, especially those about:
- size (petit, grand)
- quantity (beaucoup de, plusieurs)
- beauty (beau, joli)
- age (jeune, vieux)
- goodness (bon, mauvais)
Petit / petite is one of these, so:
- une petite grotte (adjective before the noun)
Adjectives after the verb être
When an adjective describes the subject via the verb être, it normally comes after the verb:
- l'air est frais et humide

So:

une petite grotte → adjective before the noun
l'air est frais et humide → adjectives after est

In où l'air est frais et humide, what is où doing? It’s not a question, so why use où?

Here où is a relative pronoun, not a question word.

It links two ideas:

une petite grotte
l'air est frais et humide

You can think of it as:

a small cave *where the air is cool and damp*

In English, where can also be a relative word:

the place *where I live*

Similarly, in French:

une grotte où l'air est frais et humide
= a cave where the air is cool and damp

So où is connecting the cave with the description of the air inside it.

Could we say dans laquelle l'air est frais et humide instead of où l'air est frais et humide?

Yes, grammatically you can say:

…dans une petite grotte, dans laquelle l'air est frais et humide.

But:

où is more natural and common in everyday speech and writing.
dans laquelle is more formal and heavier in style.

With places, French almost always prefers où when it means where. So où l'air est frais et humide is the best choice here.

Why is it l'air and not just air without any article?

French almost always needs an article (definite, indefinite, or partitive) before a noun, even when English does not.

In this sentence, we are talking about the air in that cave, a specific, identifiable air. So French uses the definite article:

l'air = the air

Also, le air would be awkward to pronounce, so French uses elision:

le air → l'air

You almost never see a bare noun like air on its own in this kind of context in French.

Why are the adjectives frais and humide singular and not plural, like frais et humides?

Because they describe l'air, which is singular.

Subject: l'air (masculine singular)
Adjectives must agree in gender and number:
- frais (masculine singular form)
- humide (same form for masculine and feminine singular)

If the subject were plural, the adjectives would be plural:

Les murs sont frais et humides.
(Here both frais and humides are plural.)

In the original sentence, there is only one thing being described (the air), so both adjectives stay singular.

What is the difference between frais and froid? Both seem to mean “cold”.

Both relate to temperature, but they feel different:

frais ≈ cool, fresh, pleasantly cool
- l'air est frais → the air is cool (comfortable, refreshing)
- It often has a positive or neutral feeling.
froid ≈ cold (often uncomfortably so)
- l'air est froid → the air is cold (may be unpleasant)
- More intense, can feel harsh.

So in a cave where the air feels cool and damp rather than freezing, frais is more precise and natural than froid.

What is the nuance of humide? How is it different from mouillé?

Both are about wetness, but they’re used differently:

humide = humid, damp, moist
- Often about air, climate, surfaces that are just slightly wet:
  - l'air est humide → the air is humid
  - un sol humide → a damp ground
mouillé(e) = wet, soaked
- Stronger, more obviously wet:
  - mes vêtements sont mouillés → my clothes are wet

In the sentence, l'air est … humide means the air is damp/humid, not literally “wet”, so humide is the right word.

Why is the subject ils and not elles? How do you choose between them?

Both ils and elles mean they, but:

ils is used:
- for a group of males
- for a mixed group (males + females)
- by default if the gender is unknown or not specified
elles is used only for a group of females.

So:

If the group entering the cave is all women: Elles entrent dans une petite grotte…
If it’s mixed, all men, or unspecified: Ils entrent dans une petite grotte…

In many narratives, ils is the default unless the author makes it clear it’s only women.

How should I pronounce the liaisons in ils entrent dans une petite grotte?

There are a couple of important liaisons and linkings:

ils entrent → [il‿zɑ̃tʁ]
- The final -s of ils links to entrent and sounds like z.
dans une → [dɑ̃‿zyn]
- The final -s of dans links to une and sounds like z.
petite grotte → usually [pətit gʁɔt]
- No liaison between petite and grotte (you do not say [pətit‿gʁɔt] with a [t] linking).

So the whole chunk flows something like:

[il‿zɑ̃tʁ dɑ̃‿zyn pətit gʁɔt u l‿ɛʁ ɛ fʁɛ e.tymid] (approximate phonetic line)