haisha ga mushiba o kezurimasu.

The dentist will drill the cavity.

Breakdown of haisha ga mushiba o kezurimasu.

• を: direct object particle
• が: subject particle
• 歯医者はいしゃ: dentist
• 虫歯むしば: a cavity; tooth decay

Questions & Answers about haisha ga mushiba o kezurimasu.

Why is が used after 歯医者 instead of は?

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.

What does を mark here?

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.

Why is the word order 歯医者が 虫歯を 削ります?