tā de zuǐ hěn xiǎo.

的 marks possession or association. Here, 他的嘴 means his mouth.

A simple way to see it:

• 他 = he / him
• 的 = possessive marker
• 嘴 = mouth

So 他 + 的 + 嘴 = his mouth

It often works a bit like English 's or of.

Because 小 is an adjective, and in Mandarin adjectives can act as the predicate by themselves.

So Mandarin often says:

• 嘴很小 = the mouth is small

not:

• 嘴是小 ❌

You usually use 是 with nouns, not with simple adjective predicates.

For example:

• 他是老师。 = He is a teacher.
• 他很高。 = He is tall.

So in 他的嘴很小, 小 is doing the describing directly.

In sentences like this, 很 is often not as strong as English very. It frequently acts as a natural link before an adjective.

So:

• 他的嘴很小 sounds natural and neutral.

If you say:

• 他的嘴小

it can sound more contrastive, as if you mean:

• His mouth is small (compared with something/someone else).

So although 很 can mean very, in many basic descriptive sentences it is often just part of the normal pattern.

Sometimes yes, but not always.

In this sentence, 很小 could mean:

• small
• quite small
• very small

The exact strength depends on context and intonation.

For beginners, it is helpful to remember:

• dictionary meaning of 很 = very
• actual use in many simple descriptions = often weaker than English very

So you should not always force a strong very when translating.

Yes, you can, but the feeling changes.

他的嘴小 is grammatical, but it often sounds like:

• a comparison
• a contrast
• a more literary or clipped style

For example:

• 他的嘴小，眼睛大。 = His mouth is small, but his eyes are big.

If you are just making a plain descriptive statement, 他的嘴很小 is the safer and more natural choice.

Yes, in spoken Chinese that can happen, especially with:

• family relationships
• body parts
• things closely associated with a person

So 他嘴很小 can be heard in casual speech.

However:

• 他的嘴很小 is clearer
• 他的嘴很小 is a very standard textbook-style sentence

For a learner, keeping 的 is usually best unless you are already comfortable with when omission sounds natural.

It is pronounced:

tā de zuǐ hěn xiǎo

A few helpful notes:

• 他 tā: first tone, high and level
• 的 de: neutral tone, light and unstressed
• 嘴 zuǐ: third tone
• 很 hěn: third tone
• 小 xiǎo: third tone

In natural speech, when several third tones come together, their pronunciation changes somewhat. So 嘴很小 is often said more like:

zuí hén xiǎo

That is a normal tone sandhi effect.

These two words can be tricky for English speakers.

嘴 zuǐ

• z in pinyin is not like English z in zoo
• it sounds more like ds at the start
• ui here is really pronounced more like uei

So zuǐ is roughly like dzway, but with a Mandarin third tone.

小 xiǎo

• x in pinyin is not English x
• it is a soft hissy sound made with the tongue raised forward
• it is somewhat like a very light sh, but farther forward

So xiǎo is roughly like shyao, but that is only an approximation.

If you want, the most important thing is to avoid:

• pronouncing x like ks
• pronouncing z like the English z in zebra

In writing, 他 normally means he.

But in spoken Mandarin:

• 他 tā
• 她 tā
• 它 tā

all sound the same: tā

So if you only hear the sentence, you need context to know whether the speaker means:

• he
• she
• it

In the written sentence you gave, it is specifically 他, so it means he / his.

That is just a common Mandarin way of describing things.

Mandarin often uses a topic + description style:

• 他的嘴很小
• literally: his mouth is small

English often prefers:

• He has a small mouth

Both express the same idea, but the structure is different.

This is very common in Mandarin when talking about body parts or features, such as:

• 他的眼睛很大。 = His eyes are big.
• 她的头发很长。 = Her hair is long.

So the Chinese sentence is completely natural.