Tóc cô ấy dài.

Because Vietnamese often does not use a copula like English is before adjectives.

So:

• Tóc cô ấy dài = Her hair is long
• not Tóc cô ấy là dài

Using là here would sound unnatural. In Vietnamese, adjectives like dài (long) can directly act as the predicate.

This is very common:

• Nhà lớn = The house is big
• Trời nóng = The weather is hot

So in this sentence, dài works a bit like a stative verb: to be long.

Vietnamese usually puts the possessor after the noun.

So:

• tóc cô ấy = her hair
• literally: hair she

This word order is normal in Vietnamese:

• nhà tôi = my house
• sách anh ấy = his book
• tay cô ấy = her hand

So tóc cô ấy dài is structured as:

• tóc cô ấy = her hair
• dài = is long

In this sentence, cô ấy means she or her, depending on the role in the sentence.

Here it is part of tóc cô ấy, so it is understood as her.

A few details:

• cô is originally a kinship/social term
• it can refer to a woman, often somewhat younger than one’s parents, or be used in other social relationships
• ấy means something like that, and together cô ấy becomes a common textbook way to say she/her

So learners often see:

• anh ấy = he/him
• cô ấy = she/her

In real life, Vietnamese pronouns depend a lot on age, relationship, and social context, so speakers may use a different term or a person’s name instead.

Yes. Tóc của cô ấy dài is grammatical and means the same thing.

• tóc cô ấy dài
• tóc của cô ấy dài

The version without của is very common, especially with close relationships like:

• body parts
• family members
• personal belongings in obvious contexts

So tóc cô ấy sounds natural and concise.

Using của can sound a bit more explicit or formal, but it is not required here.

It is an adjective, but in Vietnamese adjectives can often behave like predicates without any extra verb.

So in English, you need:

• Her hair is long

But in Vietnamese, you can simply say:

• Tóc cô ấy dài

This is why learners sometimes hear that Vietnamese adjectives behave a little like stative verbs. That is a useful way to think about them, even though dài is still basically an adjective meaning long.

Because the sentence is built like this:

• Subject: tóc cô ấy = her hair
• Predicate: dài = is long

So the adjective comes after the subject, just as the predicate comes after the subject in English.

Compare:

• Tóc cô ấy dài = Her hair is long

This is different from using an adjective directly inside a noun phrase:

• tóc dài = long hair

So:

• tóc dài = noun phrase
• tóc cô ấy dài = full sentence

Yes. Tóc dài often means long hair as a noun phrase.

For example:

• Cô ấy có tóc dài = She has long hair

But depending on context, Tóc dài could also be understood as a short sentence meaning Hair is long or The hair is long. Context decides.

That is why Tóc cô ấy dài is clearer as a full sentence: it clearly says that her hair is long.

Yes, absolutely.

• Tóc cô ấy dài = Her hair is long
• Cô ấy có tóc dài = She has long hair

Both are natural, but the focus is slightly different:

• Tóc cô ấy dài focuses on her hair
• Cô ấy có tóc dài focuses on her

So if you want the person to be the grammatical subject, Cô ấy có tóc dài is a very common alternative.

Usually, that would not be the best choice if you mean plain her hair is long.

Why?

Because cô by itself often has a social meaning such as:

• aunt
• miss
• female teacher
• a form of address for a woman in a certain relationship

So cô ấy is clearer as she/her.

In real conversation, Vietnamese often avoids pronouns like English does and may use:

• a name
• another kinship term
• context alone

But for a learner, cô ấy is the safe and clear form here.

No. Vietnamese does not mark tense on the verb/adjective the way English does.

So Tóc cô ấy dài by itself is neutral and usually understood from context.

Depending on context, it could mean:

• Her hair is long
• Her hair was long
• Her hair will be long

If you want to make time clear, you add time words or markers, for example:

• Bây giờ tóc cô ấy dài = Now her hair is long
• Trước đây tóc cô ấy dài = Before, her hair was long

No. You do not need a classifier here.

Tóc is a noun meaning hair, and in this sentence it is simply the subject noun. Classifiers are mainly used in Vietnamese when counting or specifying many countable nouns.

For example:

• một cái bàn = one table
• hai quyển sách = two books

But in Tóc cô ấy dài, there is no counting, so no classifier is needed.

A rough guide is:

• tóc — rising tone
• cô — level tone
• ấy — rising tone
• dài — falling tone

Very roughly for an English speaker:

• tóc ≈ tawk with a rising pitch
• cô ≈ koh
• ấy ≈ a short ay with a rising pitch
• dài ≈ something like yai or zai depending on dialect, with a falling pitch

A very approximate Northern pronunciation might sound like:

• tawk koh ay zai

But tone and vowel quality matter a lot, so it is best to learn this from audio rather than English spelling.