haisya ga musiba wo kezurimasu.

が marks 歯医者 as the grammatical subject: the person who is doing the action.

So in this sentence:

• 歯医者が = the dentist is the one who
• 虫歯を削ります = drills/files away the cavity

If you changed it to 歯医者は, the sentence would still be possible, but the nuance changes:

• 歯医者が虫歯を削ります。
  Focuses on who does it: The dentist does it.
• 歯医者は虫歯を削ります。
  Sounds more like a general statement about dentists: Dentists drill cavities / As for the dentist, they drill the cavity.

So が is very natural when simply stating who performs the action.

を marks the direct object of the verb, meaning the thing directly affected by the action.

Here:

• 虫歯を = the cavity is what gets drilled/filed away
• 削ります = drill/file/scrape away

So 虫歯を削ります means the dentist is acting directly on the cavity.

Japanese basic word order is usually Subject - Object - Verb.

So this sentence follows a very common pattern:

• 歯医者が = subject
• 虫歯を = object
• 削ります = verb

In English, we say The dentist drills the cavity, which is also subject-object-verb in this case, but Japanese keeps the verb at the end much more consistently.

That means learners often need to get used to waiting until the end for the action.

The reading is:

はいしゃ が むしば を けずります。

Word by word:

• 歯医者 → はいしゃ
• 虫歯 → むしば
• 削ります → けずります

A small pronunciation note:

• むしば is not むしは here, even though the second kanji is 歯. In this compound, it becomes ば.

削ります is the polite -ます form of 削る（けずる）.

The core meaning of 削る is:

• to shave
• to scrape
• to file down
• to cut away

In a dental context, it means something like:

• to drill away
• to remove by drilling
• to file away

So this is not just a general cut. It specifically suggests removing part of something by scraping, shaving, or drilling. That fits dental treatment very well.

That is a very natural question.

Yes, in real life the dentist works on the tooth, but Japanese often treats 虫歯 as the thing being removed or drilled away. So grammatically, the action targets the cavity/decay.

That is why 虫歯を削ります sounds natural:
the dentist is drilling away the cavity.

If the sentence wanted to focus more explicitly on the tooth, Japanese might use a different structure, but this sentence is perfectly normal as it is.

Japanese normally does not use articles like the or a/an.

So:

• 歯医者 could mean a dentist or the dentist
• 虫歯 could mean a cavity or the cavity

The exact meaning depends on context.

Japanese also often leaves number unspecified unless it matters. So a noun may be singular or plural depending on the situation. In this sentence, most learners will understand it as a single dentist and a cavity, but the grammar itself does not force that in the same way English often does.

削ります is the polite present/future form.

Japanese commonly uses the -ます form in:

• polite conversation
• textbook example sentences
• neutral formal statements

The plain form would be:

• 削る

So the sentence in plain form would be:

• 歯医者が虫歯を削る。

That sounds less polite and more casual or dictionary-like. Textbooks often introduce -ます forms early because they are useful in everyday polite speech.

It can mean either, depending on context.

Japanese non-past forms often cover both present and future.

So 削ります could mean:

• drills/files away
• will drill/file away

Possible interpretations include:

• a habitual statement: A dentist drills cavities
• something happening now: The dentist is drilling the cavity
• a near-future action: The dentist will drill the cavity

Context tells you which one is meant.

In this sentence, 歯医者 means dentist.

However, in everyday Japanese, 歯医者 can sometimes also refer to the dentist’s office / dental clinic, depending on context. For example:

• 歯医者に行く can mean go to the dentist or go to the dental clinic

But here, because 歯医者 is followed by が and performs the action 削ります, it clearly refers to the dentist as a person.

Yes. Normal Japanese writing usually does not put spaces between words.

So the standard written form is:

歯医者が虫歯を削ります。

Spaces are often added in beginner materials to make the structure easier to see:

• 歯医者 / が / 虫歯 / を / 削ります

That can be very helpful for learners, but it is not how Japanese is normally written in real life.