musiba ga arimasu.

が marks 虫歯 as the thing that exists.

So 虫歯があります literally means something like A cavity exists or There is a cavity.

In natural English, when talking about your own body, that often becomes I have a cavity.

This is a very common Japanese pattern:

• 時間があります = I have time
• お金があります = I have money
• 問題があります = There is a problem / I have a problem

So が is not marking a direct object here. It is marking the noun that goes with ある.

Japanese uses:

• ある / あります for things that are not alive or are treated as inanimate
• いる / います for people and animals

A cavity, 虫歯, is not a living thing, so Japanese uses あります.

For example:

• 猫がいます = There is a cat.
• 本があります = There is a book.
• 虫歯があります = There is a cavity / I have a cavity.

Japanese often leaves out information that is obvious from context.

If you are at the dentist, 虫歯があります will usually be understood as:

• I have a cavity, or
• There is a cavity

depending on who is speaking and the situation.

If you want to make it explicit, you could say:

• 私は虫歯があります。

But this is often unnecessary, and in many everyday situations it can sound less natural than simply leaving 私は out.

Also, in standard grammar explanations, 私は虫歯があります is understandable, but many native speakers prefer phrasing like:

• 虫歯があります。
• 虫歯になりました。
• 虫歯ができました。

So omission is very normal here.

Not literally.

A more literal breakdown is:

• 虫歯 = cavity / tooth decay
• が = subject marker
• あります = exists / there is

So the literal sense is closer to There is a cavity.

But Japanese often uses ある where English uses have, especially for conditions, possessions, and things associated with someone.

That is why the natural English translation is often I have a cavity.

虫歯 means cavity, tooth decay, or decayed tooth, depending on context.

It is a very common everyday word.

Breakdown:

• 虫 = insect / bug
• 歯 = tooth

Historically, many cultures imagined toothaches were caused by a tooth worm, which is why the word contains 虫. In modern Japanese, though, 虫歯 just means cavity or tooth decay. Native speakers do not usually think of an actual bug when they hear it.

Yes.

あります is the polite form of ある.

So:

• 虫歯がある。 = plain / casual
• 虫歯があります。 = polite

You would normally use あります with a dentist, teacher, stranger, or in most polite conversation.

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

So 虫歯があります can mean:

• I have a cavity
• I have cavities

The exact number depends on context.

If you want to be specific, you can add a number:

• 虫歯が一つあります。 = I have one cavity.
• 虫歯が二本あります。 = I have two cavities.

虫歯です is possible, but it means something slightly different.

• 虫歯があります focuses on the existence of a cavity.
• 虫歯です means something more like It is a cavity or, depending on context, I have a cavity / It’s tooth decay, but it is less directly tied to the there is / have structure.

If you are telling a dentist about your condition, 虫歯があります sounds very natural.

A dentist pointing to a tooth might say:

• これは虫歯です。 = This is a cavity.

So です is for an A is B statement, while あります is for existence.

The polite negative is:

• 虫歯がありません。 = I do not have a cavity / There is no cavity.

The plain negative is:

• 虫歯がない。

You may also hear:

• 虫歯はありません。

Using は instead of が in the negative can add a slight contrastive feeling, like As for cavities, there aren’t any.

It is pronounced:

mushiba ga arimasu

A rough guide:

• mu as in moo but shorter
• shi as in she
• ba as in bah
• ga as in gah
• a-ri-ma-su

A small note: in normal speech, the u at the end of -masu is often very light, so arimasu may sound a bit like arimas.

Not if you mean I have a cavity.

• 歯 means tooth / teeth
• 虫歯 means cavity / tooth decay

So:

• 歯があります = There is a tooth / I have teeth

That is a completely different meaning.

You need 虫歯 specifically to talk about a cavity.

The spaces are just for learning.

Normal Japanese is usually written without spaces:

• 虫歯があります。

Textbooks and beginner materials often insert spaces to make it easier to see the parts:

• 虫歯　が　あります。

Both represent the same sentence.