ha ni sikou ga tuite imasu.

It is read しこう.

• 歯 = tooth / teeth
• 垢 = dirt, grime, buildup

So 歯垢 means dental plaque.

に marks the place where something is attached.

So in 歯に歯垢がついています, 歯に means:

• on the teeth
• attached to the teeth

This is a very common use of に with verbs like つく when something ends up on a surface or at a location.

Because 歯垢 is the thing being described as attached.

In this sentence:

• 歯に = location
• 歯垢が = the thing that is there / attached
• ついています = is attached / is stuck

So が marks plaque as the grammatical subject of the sentence.

If you changed the topic, you could also say something like 歯垢は歯についています, but that has a different emphasis.

Here, ついています means is attached, is stuck, or is on.

It comes from the verb つく.

In this sentence, つく does not mean arrive or turn on. It means:

• to stick
• to attach
• to cling to a surface

So 歯垢がついています means that plaque is in the state of being attached to the teeth.

Because 〜ている often describes a current state resulting from a change.

• つく = to become attached
• ついている = to be attached

So:

• つきます would sound more like it attaches / it will attach
• ついています means it is attached now

That is why ついています is the natural form here.

No. Here います is just the polite ending of 〜ている.

So the sentence is built like this:

• つく
• ついて
• ついている
• ついています (polite)

This います is not the separate verb meaning to exist for living things.

It can be either, depending on context.

Japanese nouns usually do not show singular/plural the way English does. So 歯 can mean:

• a tooth
• teeth

In this sentence, English would usually translate it as teeth, because plaque is often talked about generally.

It only looks repetitive at first glance.

The first 歯 is the noun tooth/teeth.

The second 歯 is just part of the compound word 歯垢.

So this is not really tooth ... tooth ... in the same way it might feel in English. It is:

• 歯に = on the teeth
• 歯垢が = plaque

Yes, it is grammatically possible, but ついています is more natural here.

Why?

• あります focuses on existence: there is plaque
• ついています focuses on attachment to a surface: plaque is stuck/on the teeth

Since plaque is something that clings to teeth, ついています is the more specific and natural choice.

The dictionary form is つく.

A useful pair to know is:

• つく = to attach / to stick (intransitive)
• つける = to attach something / to put something on (transitive)

For example:

• 汚れがつく = dirt gets on something
• 汚れをつける = to put a stain on something

In your sentence, plaque is not being actively attached by someone, so the intransitive つく is used.

Yes, if you want a different emphasis.

For example:

• 歯には歯垢がついています。

This adds contrast or topic emphasis, something like:

• On the teeth, there is plaque
• As for the teeth, plaque is attached

The original 歯に歯垢がついています is a neutral statement. Adding は changes the focus.

Yes. ついています is polite.

A plain version would be:

• 歯に歯垢がついている。

So:

• ついている = plain
• ついています = polite

The polite form is very natural in contexts like:

• talking to a patient
• explaining something in class
• speaking generally in a careful or formal way