Vết ẩm ở trên tường.

Because Vietnamese does not always need a separate verb like English is.

Here, ở carries the idea of location, so the sentence is literally something like:

• vết ẩm = damp mark / damp stain
• ở trên tường = is on the wall

So ở is doing the job that English expresses with is/on together.

Also, in Vietnamese, short descriptive sentences often sound very compact, especially in captions, notes, or simple observations.

Vết means a mark, stain, spot, trace, or patch left on a surface.

It is a very common word for visible marks, for example:

• vết bẩn = a dirty mark / stain
• vết mực = an ink stain
• vết thương = a wound / injury mark
• vết cắt = a cut mark

So in vết ẩm, vết is the main noun, and ẩm describes what kind of mark it is.

Because in Vietnamese, descriptive words usually come after the noun.

So:

• vết ẩm = damp stain
• literally: stain damp

This is very normal Vietnamese word order. English usually puts adjectives before nouns, but Vietnamese usually puts them after.

Compare:

• nhà lớn = big house
• áo trắng = white shirt
• tường cao = tall wall

They are related, but not the same.

• ẩm = damp, moist, slightly wet, often because of humidity
• ướt = wet

For a wall, ẩm is more natural if you mean a moisture problem, seepage, or a damp patch.
If you say vết ướt, it sounds more like a mark that is actively wet right now.

So for a wall stain caused by moisture, vết ẩm is a good choice.

Not really.

• ở marks location: at/in/on
• trên specifies the position: on/above

So ở trên tường means something like located on the wall.

That said, Vietnamese often allows a shorter version:

• Vết ẩm trên tường.

This also sounds natural.
Using ở trên can make the location feel a bit more explicit.

Yes.

Both are possible:

• Vết ẩm ở trên tường.
• Vết ẩm trên tường.

The shorter version is often more natural in everyday Vietnamese, especially in simple descriptions.

If you want a very natural full sentence meaning There is a damp stain on the wall, Vietnamese speakers would very often say:

• Trên tường có một vết ẩm.
• Có một vết ẩm trên tường.

It can function as a full sentence, but it is somewhat compact.

It sounds natural in contexts like:

• a description
• a caption
• a note
• pointing something out

In more neutral, fully spelled-out speech, many speakers would prefer:

• Có một vết ẩm trên tường. = There is a damp stain on the wall.
• Trên tường có một vết ẩm. = On the wall there is a damp stain.

So your sentence is understandable and usable, but it feels a little brief.

Because Vietnamese does not use articles like English a/an/the.

A bare noun like vết ẩm can mean:

• a damp stain
• the damp stain
• damp stains

The exact meaning depends on context.

If you want to make it clearly one stain, you can add một:

• một vết ẩm = one / a damp stain

But Vietnamese often leaves this unstated when the context is already clear.

If your intended meaning is There is a damp stain on the wall, then yes, that is often the most natural everyday sentence.

Compare:

• Vết ẩm ở trên tường.
  = The damp stain is on the wall / Damp stain on the wall.
  This feels more like naming or identifying the thing and its location.

• Có một vết ẩm trên tường.
  = There is a damp stain on the wall.
  This is a very natural existential sentence.

So the best version depends on what you want to emphasize.

Usually, in this kind of sentence, it means on the wall.

The word trên can mean both on and above, depending on context. But with a mark or stain, the natural interpretation is that it is on the surface of the wall.

So vết ẩm trên tường would normally be understood as a damp stain on the wall, not floating above it.

No. It is understandable, but depending on the exact situation, Vietnamese speakers might also say:

• vết ố ẩm = a damp discoloration / damp stain
• mảng ẩm = a damp patch
• vết mốc = a mold mark
• tường bị ẩm = the wall is damp

So vết ẩm is fine, but other expressions may sound more specific depending on whether you mean moisture, discoloration, mold, or a larger patch.

The structure is:

• vết ẩm = noun phrase
• ở trên tường = location phrase

So the pattern is basically:

• [thing] + [location]

This is a very common Vietnamese pattern, for example:

• Sách ở trên bàn. = The book is on the table.
• Mèo ở dưới ghế. = The cat is under the chair.
• Vết ẩm ở trên tường. = The damp stain is on the wall.

It is a simple and useful pattern for talking about location.