Mắt của tôi đau.

A simple breakdown is:

• mắt = eye / eyes
• của = a possession marker, like of
• tôi = I / me / my
• đau = hurt / painful / ache

So the structure is roughly:

• Mắt của tôi = my eye(s)
• đau = hurt

Very literally, it is something like My eye(s) hurt or My eyes are painful.

Của marks possession, so mắt của tôi means my eye(s).

However, with body parts, Vietnamese often drops của and simply puts the possessor right after the noun:

• mắt tôi đau

This is usually more natural in everyday speech than mắt của tôi đau.

So:

• mắt của tôi đau = correct, clear
• mắt tôi đau = also correct, often more natural

Using của is not wrong; it can just sound a bit more explicit or slightly less colloquial in this kind of sentence.

Vietnamese nouns usually do not change form for singular vs. plural, so mắt can mean:

• eye
• eyes

The exact meaning depends on context.

So Mắt của tôi đau could mean:

• My eye hurts
• My eyes hurt

If you want to be specific, you can add words such as:

• một mắt = one eye
• hai mắt = two eyes / both eyes
• mắt trái = left eye
• mắt phải = right eye

For example:

• Mắt trái của tôi đau = My left eye hurts

In Vietnamese, words like đau often come after the body part or thing being described.

So the pattern is commonly:

• body part + đau

Examples:

• đầu đau = the head hurts
• tay đau = the arm/hand hurts
• mắt đau = the eye hurts

In English, we often say My eye hurts, where hurts is a verb. In Vietnamese, đau can behave like a stative verb or adjective-like predicate, so it naturally appears after the noun phrase.

It is understandable and grammatically fine, but native speakers often prefer more natural alternatives such as:

• Mắt tôi đau.
• Tôi đau mắt.
• Tôi bị đau mắt.

These have slightly different feels:

• Mắt tôi đau. = My eye(s) hurt.
• Tôi đau mắt. = I have eye pain / My eyes hurt.
• Tôi bị đau mắt. = often sounds like I have sore eyes / I’m suffering from an eye problem

So the sentence you were given is correct, but in everyday speech, Mắt tôi đau or Tôi đau mắt may be more common.

Both are natural, but the focus is slightly different.

• Mắt tôi đau puts the focus on my eye(s)
• Tôi đau mắt puts the focus on me, and says I am experiencing eye pain

Compare:

• Mắt tôi đau. = My eye hurts.
• Tôi đau mắt. = I have eye pain / My eyes hurt.

In many situations, they are practically interchangeable.

Yes. That is one reason Vietnamese can feel different from English here.

Đau can express the idea of:

• to hurt
• to ache
• painful
• in pain

So depending on the structure, English may translate it in different ways.

Examples:

• Tay tôi đau. = My arm hurts.
• Tôi đau đầu. = I have a headache / My head hurts.

It is best not to force a single English part of speech onto đau every time.

A rough guide is:

• mắt — short vowel, with a sharp falling tone
• của — starts a bit like koo-uh with the tone on của
• tôi — roughly like toy with the level tone
• đau — roughly like dow with the level tone

A very rough English-style approximation is:

• mut kwa toy dow

But that is only approximate. The tones and vowels matter a lot in Vietnamese, especially for mắt and của.

Yes. Since mắt can be singular or plural, context decides.

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

• Một mắt của tôi đau. = One of my eyes hurts.
• Hai mắt tôi đau. = My two eyes hurt.
• Cả hai mắt tôi đều đau. = Both my eyes hurt.

Without that extra information, listeners usually understand the intended meaning from the situation.

Yes, very often, especially when the owner is obvious from context.

For example, if you are talking about yourself, you may hear:

• Đau mắt quá. = My eyes hurt so much.
• Mắt đau quá. = My eye(s) hurt so much.

Vietnamese often relies more on context than English does. Still, including tôi is perfectly normal when needed for clarity.

A few natural ways are:

• Mắt của tôi đau à?
• Mắt tôi đau à?
• Mắt tôi có đau không?

These can mean things like:

• Does my eye hurt?
• Is my eye hurting?
• Are my eyes hurting?

Very commonly, Vietnamese uses không for yes-no questions:

• Mắt tôi có đau không?

You can say:

• Mắt tôi không đau. = My eye doesn’t hurt.
• Mắt của tôi không đau. = My eye(s) don’t hurt.

Here, không goes before đau.

Pattern:

• noun phrase + không + đau

Example:

• Mắt tôi không đau nữa. = My eye doesn’t hurt anymore.