Learn Hardcore French: Sa ceinture tombe parce que son jean est trop large. - Her belt falls because her jeans are too loose.

English uses a plural form (jeans) even for one pair, but French uses a singular form in the same situation. So son jean corresponds to his/her jeans in English.

How do we know that ceinture is feminine and jean is masculine?

Unfortunately, French noun gender is mostly arbitrary and must be learned with each word.

You should learn: une ceinture (belt) → feminine
And: un jean (jeans) → masculine

A good habit: memorize nouns with their article:

une ceinture
un jean

Over time you will develop intuition, but there is no simple rule that would predict these genders.

Why is the verb form tombe and not something like tomber or tombent?

Tomber is the infinitive (to fall). To match the subject sa ceinture (third person singular), we conjugate it in the present tense:

je tombe – I fall
tu tombes – you fall
il/elle tombe – he/she/it falls
nous tombons – we fall
vous tombez – you fall
ils/elles tombent – they fall

The subject here is la ceinture (feminine singular), so we use elle tombe → sa ceinture tombe.
Tomber (infinitive) would be wrong in a normal sentence; tombent would be for a plural subject.

In English we say is falling down. Why is French just tombe and not something like est tombant?

French usually uses the simple present to cover both:

English simple present: falls
English present continuous: is falling

So sa ceinture tombe can mean:

Her/his belt is falling down (right now)
or
Her/his belt falls down (regularly)

The construction est tombant is not natural in French for this meaning. The progressive form (is falling) is much less common in French and is expressed differently (often just with the simple present).

Why is it parce que and not car or puisque?

All three can introduce a reason, but they differ slightly:

parce que: neutral, most common for because.
- Sa ceinture tombe parce que son jean est trop large.
car: more formal/literary, often used in writing.
- Sa ceinture tombe, car son jean est trop large.
puisque: implies the reason is already known or obvious to both speakers, closer to since/as.
- Sa ceinture tombe, puisque son jean est trop large.

In basic spoken French, parce que is the default choice for because.

What exactly does trop large mean here? Is it too big, too wide, or something else?

In clothing, large generally means wide/loose rather than tall.

trop large: too wide, too loose, too baggy

By contrast:

trop grand: too big in size overall (often including length/height)

In this context:

son jean est trop large suggests the jeans are too loose around the waist/hips, so the belt falls.

Why doesn’t large change form here? Shouldn’t it agree with something?

Adjectives in French agree with the noun they describe in gender and number.

The noun is jean: masculine singular.
So the adjective stays masculine singular: large.

Forms of large:

Masculine singular: large (un jean large)
Feminine singular: large (une ceinture large)
Masculine plural: larges (des jeans larges)
Feminine plural: larges (des ceintures larges)

In your sentence, son jean = masculine singular → trop large (no added s).

Why does the adjective large come after jean and not before it?

Most French adjectives normally come after the noun:

un jean large = wide/loose jeans
une ceinture noire = a black belt

Only some common adjectives (often short ones like beau, grand, petit, jeune, vieux, etc.) usually come before the noun.

Large does not belong to this special group, so it follows the noun:

son jean est trop large (not son large jean).

Can this sentence mean his belt as well as her belt?

Yes. French possessives (son, sa) do not distinguish between his and her. They only show the gender/number of the thing owned:

sa ceinture = his belt / her belt
son jean = his jeans / her jeans

To know whether it’s his or her, you need context (who you are talking about).

How do you pronounce the parts of this sentence?

Approximate pronunciation (in rough English sounds):

sa → [sa] (like sah)
ceinture → [sɛ̃.tyʁ] (roughly san-tewr, with a nasal in sound in cein)
tombe → [tɔ̃b] (roughly tomb, but final e silent, nasal on)
parce que → [paʁs kə] (roughly parskə, the e in que is a very short schwa)
son → [sɔ̃] (nasal on sound, like sohn)
jean → [ʒɑ̃] (like the name Jean in French; j sound like the s in measure)
est → [ɛ] (like eh)
trop → [tʁo] (like troh, final p silent)
large → [laʁʒ] (roughly larj, with the French r)

Full sentence:
Sa ceinture tombe parce que son jean est trop large.
→ [sa sɛ̃tyʁ tɔ̃b paʁs kə sɔ̃ ʒɑ̃ ɛ tʁo laʁʒ]

Could we say son pantalon instead of son jean? Would it change the meaning?

Yes, grammatically it’s fine:

Sa ceinture tombe parce que son pantalon est trop large.

Pantalon is trousers/pants in general, while jean specifically means jeans (denim trousers).
So the structure and grammar stay the same; only the type of clothing changes.