siseki ga ooi desu.

歯石 is dental tartar or calculus: the hard mineral buildup that forms on teeth.

• 歯 = tooth
• 石 = stone

So the kanji literally look like tooth-stone, which matches the idea of hardened buildup on teeth.

It is read:

しせき が おい です? No—careful: しせき が おおい です

More naturally written in kana:

しせき が おおい です。

A rough pronunciation guide:

• しせき = shi-se-ki
• が = ga
• おおい = oo-i, with a long o
• です = desu

So the key thing to notice is that 多い is おおい, not おい.

が marks 歯石 as the thing being described.

So the structure is basically:

歯石 + が + 多い
tartar + subject marker + is abundant / is a lot

In English, we often say There is a lot of tartar or You have a lot of tartar, but Japanese often just marks the thing itself with が and describes its state.

多い means many or a lot.

Here it means that the amount of tartar is large, so the whole sentence means something like:

• There is a lot of tartar.
• You have a lot of tartar buildup.

Even though 多い is often translated as many or much, in Japanese it behaves as an i-adjective.

Both can express a lot, but they work a little differently.

• 多い is an i-adjective
• たくさん is usually an adverb-like word / noun-like word

So:

• 歯石が多いです = There is a lot of tartar.
• 歯石がたくさんあります = There is a lot of tartar.

Both are understandable.
歯石が多いです is a simple descriptive sentence.
歯石がたくさんあります focuses more on existence: there exists a lot of tartar.

多い is already a complete adjective, and です is added to make the sentence polite.

So:

• 歯石が多い。 = plain / casual
• 歯石が多いです。 = polite

This is a very common pattern with i-adjectives:

• 高いです
• 忙しいです
• 多いです

So です here is not changing the core meaning; it is mainly adding politeness.

Yes, possibly—but it is omitted because Japanese often leaves out information that is clear from context.

For example, a dentist looking at your teeth might say:

歯石が多いです。

And the understood meaning would be:

• You have a lot of tartar.

But the sentence itself does not explicitly say you. Japanese often relies on context instead of stating the subject directly.

Yes, but it changes the nuance.

• 歯石が多いです = The tartar is abundant / There is a lot of tartar.
• 歯石は多いです = As for tartar, there is a lot of it.

が is more neutral for simply describing what is present.
は adds a topic feeling, and can sometimes sound contrastive depending on context.

If you are just learning the basic sentence, 歯石が多いです is the safer form to understand first.

Yes, it is understandable and natural enough, especially in a dental context.

That said, depending on the situation, native speakers might also say things like:

• 歯石が多いですね。
  There is a lot of tartar, isn’t there.

• 歯石がたくさん付いています。
  A lot of tartar has built up / is attached.

• 歯石が結構あります。
  There is quite a bit of tartar.

So 歯石が多いです is fine, but in real-life conversation, a dentist may use a slightly fuller expression.

Japanese often expresses ideas differently from English.

English says:

• You have a lot of tartar.

Japanese often says:

• Tartar is a lot.
• A lot of tartar exists.

So instead of using a direct equivalent of have, Japanese may describe the thing itself with 多い, or use あります to say that it exists.

This is a very common pattern in Japanese and something English speakers have to get used to.

多い is an i-adjective.

That means it behaves like words such as:

• 高い = expensive / high
• 早い = early / fast
• 少ない = few / little
• 多い = many / much

So in this sentence, 多い is the main descriptive word, not a verb.

In casual speech, you can simply drop です:

歯石が多い。

That sounds plain and direct.

Examples:

• polite: 歯石が多いです。
• casual: 歯石が多い。

Here are the common forms of 多い:

• present affirmative: 歯石が多いです。
  There is a lot of tartar.

• present negative: 歯石が多くないです。
  There is not much tartar.

• past affirmative: 歯石が多かったです。
  There was a lot of tartar.

• past negative: 歯石が多くなかったです。
  There was not much tartar.

This is the normal conjugation pattern for an i-adjective.

Yes, but the structure changes.

In your sentence, 多い is the predicate:

• 歯石が多いです。
  There is a lot of tartar.

If 多い directly modifies a noun, it comes before that noun:

• 歯石が多い人
  a person who has a lot of tartar

So learners should notice the difference between:

• X が 多い = X is abundant / there is a lot of X
• 多い + noun = many/a lot of noun