Có một dấu trên hình.

Here có means there is / there are, not to have.

In Vietnamese, có can do both jobs:

• Tôi có một cái bút. = I have a pen.
• Có một cái bút trên bàn. = There is a pen on the table.

In Có một dấu trên hình, it introduces the existence of something: There is a mark/sign on the picture.

So the pattern is:

có + thing + location

• Có một dấu trên hình.
• Có một con mèo dưới bàn.

Một literally means one, but very often it also works like a / an in English.

So một dấu can mean:

• one mark
• a mark

Which meaning is intended depends on context. In this sentence, it most naturally feels like a mark or one mark.

Vietnamese does not have a direct equivalent of the English articles a/an/the, so một often helps introduce a single countable thing.

Dấu is a flexible word. Depending on context, it can mean:

• mark
• sign
• symbol
• accent mark / diacritic
• trace

In Có một dấu trên hình, the safest general meaning is a mark or a sign on the picture/image.

Because you said the meaning is already shown to the learner, the key point is that dấu is a broad noun, and the exact English translation depends a lot on context.

Good question. Vietnamese often uses classifiers, but not always.

You may hear both:

• một dấu
• một cái dấu

For many nouns, especially in shorter or more neutral phrasing, the classifier can be omitted. With dấu, omitting it is very natural.

Very roughly:

• một dấu = a concise, neutral way to say a mark/sign
• một cái dấu = also possible, a bit more explicit or colloquial in some contexts

So Có một dấu trên hình is perfectly normal.

Trên hình means on the picture / on the image / in the picture, depending on context.

Breakdown:

• trên = on, above, over
• hình = picture, image, figure, sometimes shape

So:

• trên hình = on the image/picture
• in natural English, this may sometimes be translated as in the picture, even though Vietnamese uses trên

That is common when talking about something printed, drawn, or visible on an image.

Vietnamese often uses an existential structure that works like this:

Có + noun phrase + location

So:

• Có = there is/are
• một dấu = a mark
• trên hình = on the picture

Literally, it is something like:

There exists a mark on the picture.

This is a very common Vietnamese pattern for introducing something that exists somewhere.

Yes, absolutely.

Both are natural:

• Có một dấu trên hình.
• Trên hình có một dấu.

The difference is mainly about focus:

• Có một dấu trên hình starts by presenting existence: There is a mark...
• Trên hình có một dấu starts with the location: On the picture, there is a mark.

Vietnamese often allows this kind of flexibility.

Because this sentence is about existence, not identity.

Use là when you are saying that one thing is another thing:

• Đây là một dấu. = This is a mark.

Use có when you are saying that something exists / is present somewhere:

• Có một dấu trên hình. = There is a mark on the picture.

So là and có are doing different jobs.

Yes. Hình is another flexible word. It can mean:

• picture
• image
• figure
• shape

The correct translation depends on context.

For example:

• trong hình = in the picture / in the image
• hình tròn = circle or literally round shape
• hình học = geometry

In your sentence, hình most likely means picture/image.

Yes, but the meaning changes slightly.

• Có một dấu trên hình = There is a mark / one mark on the picture
• Có dấu trên hình = There is marking on the picture / there are marks on the picture / the picture has a mark on it, depending on context

Without một, the sentence can sound less specifically countable and more general.

So if you want to clearly say a single mark, keeping một is better.

The tones matter a lot in Vietnamese. A rough guide:

• có: rising tone
• một: heavy, low glottalized tone
• dấu: dipping/creaky tone
• trên: level tone
• hình: low tone

So the full sentence has a noticeable melody. Learners often need extra practice with:

• một
• dấu
• hình

because their tones are easy to flatten or mix up.

A practical tip: learn the sentence in chunks rather than word by word:

• Có một dấu
• trên hình

That usually helps pronunciation sound more natural.