tumetai mizu ga ha ni simimasu.

A natural reading is:

つめたい みず が は に しみます。
Tsumetai mizu ga ha ni shimimasu.

A few notes:

• 冷たい = つめたい
• 水 = みず
• 歯 = は
• しみます = shimimasu

Even though 歯 is written with the kana は, here it is read ha, not wa. Only the particle は is read wa.

In this sentence, しみます means something like:

• sting
• hurt sharply
• send a sharp sensation into
• be sensitive to

So 冷たい水が歯にしみます means that cold water causes a sharp, unpleasant feeling in the teeth.

This is a very common Japanese expression for tooth sensitivity. It is not the same as just general pain like 歯が痛い (my tooth hurts). しみる / しみます specifically suggests a sharp sensation caused by something like cold, heat, sweetness, etc.

Because 水 is treated as the thing causing the sensation.

So the structure is roughly:

• 冷たい水が = cold water is the thing that does it
• 歯に = to the teeth
• しみます = stings / is painful

Japanese often builds this kind of sentence around what causes the effect, so が marks 冷たい水 as the grammatical subject.

In English, we might focus more on my teeth are sensitive to cold water, but Japanese often says it more like cold water stings my teeth.

Here, に shows the target or point affected by the action/sensation.

So:

• 歯にしみます = it stings in the teeth / it affects the teeth sharply

You can think of に here as marking where the sensation hits.

This use is common with しみる:

• 目にしみる = it stings the eyes
• 歯にしみる = it stings the teeth

So に is not exactly in or to in a simple one-word way, but it marks the place/person affected.

歯がしみます is also possible, but it means something slightly different in focus.

• 冷たい水が歯にしみます
  = Cold water stings my teeth.
  Focus: the cold water is causing the effect.

• 歯がしみます
  = My teeth are sensitive / my teeth sting.
  Focus: the teeth are what hurt.

So your sentence highlights the trigger, not just the condition of the teeth.

Japanese often leaves out possession when it is obvious from context.

So 歯 here naturally means my teeth, even though 私の is not stated.

Saying 私の歯にしみます is grammatically possible, but it usually sounds unnecessary unless you are emphasizing that it is your teeth, not someone else’s.

This is very common with body parts in Japanese:

• 頭が痛い = My head hurts
• 手を洗う = Wash my hands
• 歯にしみる = It stings my teeth

Japanese nouns usually do not change form for singular vs. plural.

So 歯 can mean:

• tooth
• teeth

The exact meaning depends on context.

In this sentence, English naturally says teeth, but Japanese just uses 歯 without needing a plural form.

Because Japanese adjectives usually come before the noun they describe.

So:

• 冷たい水 = cold water

This is just like:

• 大きい犬 = big dog
• 新しい車 = new car

Also, 冷たい is specifically used for things that are physically cold to the touch, like water or ice.

Normally, no.

Japanese distinguishes between:

• 冷たい = cold things you touch or feel physically, like water, drinks, metal
• 寒い = cold weather / cold air / feeling cold because of temperature

So:

• 冷たい水 = correct
• 寒い水 = unnatural in normal Japanese

Use 冷たい for water.

The plain/dictionary form is しみる.

So the sentence in plain style would be:

冷たい水が歯にしみる。

Compare:

• しみます = polite
• しみる = plain

Both mean the same basic thing. The difference is politeness and style.

Yes, it is natural and very understandable.

It sounds like something you might say when talking about tooth sensitivity.

Depending on context, native speakers might also say:

• 冷たいものが歯にしみます。 = Cold things sting my teeth.
• 歯がしみます。 = My teeth are sensitive / sting.
• 冷たい水を飲むと歯にしみます。 = When I drink cold water, it stings my teeth.

Your original sentence is correct and natural.

It could use は, but that would change the nuance.

Your sentence:

• 冷たい水が歯にしみます。

This presents cold water as the thing causing the sensation.

If you said:

• 冷たい水は歯にしみます。

that would sound more like:

• As for cold water, it stings my teeth
• or it might contrast cold water with something else

For example, it can imply something like:

• Hot tea is fine, but cold water stings my teeth

So が is the more neutral choice here.

No. しみる has several related uses, all connected to something soaking in, penetrating, or affecting something deeply.

Common examples:

• 目にしみる = sting the eyes
• 歯にしみる = sting the teeth
• 身にしみる = feel deeply / sink in emotionally or physically
• 雨が服にしみる = rain soaks into clothes

So in your sentence, the idea is that the coldness penetrates the teeth and causes a sharp feeling.

Yes. Normal Japanese is usually written without spaces:

冷たい水が歯にしみます。

The spaced version is often used for learners to make the parts easier to see:

冷たい　水　が　歯　に　しみます。

Both represent the same sentence.