Tôi biết tuổi của mẹ.

Vietnamese usually follows subject + verb + object, just like English:

• Tôi = I
• biết = know
• tuổi của mẹ = mother’s age / the age of mother

So the sentence is literally structured as:

• I + know + age of mother

What feels different is the possessive part. English often puts the possessor first (my mother’s age), but Vietnamese commonly says:

• tuổi của mẹ = the age of mother

So the sentence sounds like I know the age of my mother in a word-for-word sense.

Của marks possession or an of relationship.

In this sentence:

• tuổi = age
• của mẹ = of mother

So tuổi của mẹ means mother’s age or the age of mother.

A very useful pattern is:

• noun + của + person/thing

Examples:

• nhà của tôi = my house
• xe của anh ấy = his car
• tên của cô ấy = her name

So here:

• tuổi của mẹ = mother’s age

In Vietnamese, family terms often work differently from English. Mẹ by itself can already mean my mother when the speaker’s point of view is clear.

So in many contexts:

• mẹ = my mother
• ba/bố = my father
• anh = my older brother / an older male depending on context
• chị = my older sister / an older female depending on context

That means Tôi biết tuổi của mẹ is naturally understood as I know my mother’s age.

If you want to make it extra explicit, you can say:

• Tôi biết tuổi của mẹ tôi.

That is clearer, but sometimes a bit heavier than necessary if the relationship is already obvious.

It is understandable and grammatically fine, but it sounds a little formal or bookish.

In everyday conversation, many speakers would more naturally say:

• Tôi biết mẹ bao nhiêu tuổi.

Literally, that is closer to I know how old my mother is.

This version feels more conversational because Vietnamese often expresses age with bao nhiêu tuổi rather than with the noun tuổi alone.

So:

• Tôi biết tuổi của mẹ = correct, a bit formal
• Tôi biết mẹ bao nhiêu tuổi = very natural in speech

They are very close in meaning, but the phrasing is different.

1. biết tuổi của mẹ

   • literally know mother’s age
   • uses tuổi as a noun
   • slightly more formal or written

2. biết mẹ bao nhiêu tuổi

   • literally know mother how many years old
   • asks or states age in the more common Vietnamese way
   • more natural in everyday speech

English often uses both patterns too:

• I know my mother’s age
• I know how old my mother is

Vietnamese has the same kind of choice here.

Usually, learners should keep của here.

• Tôi biết tuổi của mẹ = clear and standard
• Tôi biết tuổi mẹ = may sound incomplete, unnatural, or stylistically marked depending on context

Vietnamese sometimes omits của in certain fixed or compact expressions, but that is not the safest pattern for a learner. In ordinary neutral Vietnamese, tuổi của mẹ is the better form.

So if you are unsure, use:

• noun + của + possessor

It can do both, depending on context.

In this sentence, biết means know a fact/information:

• Tôi biết tuổi của mẹ. = I know my mother’s age.

But biết can also mean know/be acquainted with:

• Tôi biết cô ấy. = I know her.

So biết is broader than just one English meaning. Context tells you whether it means:

• knowing information
• being familiar with someone or something
• having learned how to do something in some cases

Here it clearly means to know information.

Vietnamese does not use articles the same way English does.

There is no direct equivalent of the in this sentence, and possession is handled differently. Instead of saying my mother’s age with a possessive adjective like my, Vietnamese uses:

• the kinship word itself: mẹ
• or a possession structure: của

So Vietnamese can express the idea without separate words matching the and my one-for-one.

That is very common. Vietnamese often leaves things to context that English must state explicitly.

In principle, yes, but in this sentence most people will understand mẹ as my mother unless context says otherwise.

Vietnamese kinship terms are highly context-based. Depending on the situation, mẹ could refer to:

• my mother
• your mother
• his/her mother
• mother as a form of address in some family contexts

But with Tôi biết tuổi của mẹ, the default reading is usually I know my mother’s age.

If you want to remove ambiguity, you can say:

• Tôi biết tuổi của mẹ tôi. = I know my mother’s age.
• Tôi biết tuổi của mẹ anh ấy. = I know his mother’s age.

Yes, tuổi is the basic Vietnamese noun for age.

Examples:

• tuổi của tôi = my age
• bao nhiêu tuổi = how old / how many years old
• mấy tuổi = how old (often for children or in casual speech)

But when Vietnamese talks about someone’s age, it very often uses question-like structures such as:

• Mẹ bao nhiêu tuổi? = How old is your/my mother?
• Tôi biết mẹ bao nhiêu tuổi. = I know how old my mother is.

So even though tuổi means age, Vietnamese does not always use it in exactly the same kinds of noun phrases English prefers.

A rough breakdown is:

• Tôi — mid-level tone
• biết — rising then cut/glottalized tone in the North
• tuổi — dipping/rising tone with a glottal quality in the North
• của — falling tone
• mẹ — heavy low falling tone

If you are learning Northern Vietnamese, pay special attention to the contrast between:

• biết
• tuổi
• mẹ

because their tones are different and important for being understood.

A rough English-style approximation is not very reliable, so it is best to hear native audio and imitate it chunk by chunk:

• Tôi biết | tuổi của mẹ.

It may be understood, but it is not the best standard form for a learner.

Safer options are:

• Tôi biết tuổi của mẹ tôi.
• Tôi biết mẹ tôi bao nhiêu tuổi.

These are clearer and more natural.

In general, when you build a possession phrase carefully, it is better to include của unless you are already comfortable with when Vietnamese can omit it naturally.

A very natural everyday version is:

• Tôi biết mẹ bao nhiêu tuổi.

If you want to be extra explicit:

• Tôi biết mẹ tôi bao nhiêu tuổi.

So a good rule is:

• for a textbook-style possession phrase: Tôi biết tuổi của mẹ.
• for natural spoken Vietnamese: Tôi biết mẹ bao nhiêu tuổi.