chungchiga saenggiji anhge haryeomyeon bamedo chisireul sseuneun ge johayo.

Literally, it means cavities form / develop.

In Korean, 생기다 often means to arise, occur, develop, come into being. So:

• 충치가 생기다 = cavities develop
• 문제가 생기다 = a problem arises
• 여드름이 생기다 = pimples appear

So this sentence is phrased as making sure cavities don’t develop, rather than using a verb like avoid directly.

Because 생기다 is an intransitive verb. The cavity is the thing that comes into existence / develops, so it takes the subject marker 가.

• 충치가 생기다 = a cavity develops
• not 충치를 생기다

If you see 가/이 with health conditions or problems, that is very common in Korean:

• 감기가 걸리다 = to catch a cold
• 열이 나다 = a fever comes out / to have a fever
• 문제가 생기다 = a problem arises

It breaks down like this:

• 생기다 = to develop
• 생기지 않다 = to not develop
• 생기지 않게 = so that it does not develop / in a way that it does not develop

Here, -게 turns the preceding part into something like so that... or in such a way that...

So:

• 충치가 생기지 않게 = so that cavities don’t form

This pattern is very common:

• 잊지 않게 메모하세요 = Make a note so that you don’t forget.
• 다치지 않게 조심하세요 = Be careful so that you don’t get hurt.

Because the full expression is really:

• 충치가 생기지 않게 하다

That means to make it so that cavities do not develop, which is a natural Korean way to say to prevent cavities.

Then -려면 attaches to 하다:

• 하려면 = if you want to do that / if you are going to do that

So:

• 충치가 생기지 않게 하려면
• literally: if you want to make it so that cavities don’t develop
• natural English: if you want to prevent cavities

Yes, that is also natural.

There is a small nuance difference:

• 충치가 생기지 않으려면 = if you don’t want cavities to develop / if you want to avoid getting cavities
• 충치가 생기지 않게 하려면 = if you want to prevent cavities from developing

The sentence you were given sounds a bit more active/preventive, as if you are taking steps to stop something from happening.

So both are possible, but 생기지 않게 하려면 emphasizes prevention a little more clearly.

• 밤에 = at night
• 밤에도 = at night too / also at night / even at night

The particle 도 adds the meaning of also, too, even.

So the sentence is not just saying use floss at night. It suggests something like:

• use floss at night too
• even at night, flossing is good

Depending on context, it may imply that flossing should not be limited to only one time of day.

Because 쓰다 can mean to use, not just to write.

Korean often uses 쓰다 for tools, products, or methods:

• 약을 쓰다 = use medicine
• 컴퓨터를 쓰다 = use a computer
• 치실을 쓰다 = use dental floss

So 치실을 쓰다 is a normal, natural expression.

You may also hear:

• 치실을 사용하다 = use dental floss

That sounds a bit more formal than 쓰다.

This is a very common pattern:

• V-는 게 좋아요 = it’s good to do V / you should do V

Here:

• 쓰는 = using
• 것이 = the thing that...
• 게 = a spoken contraction of 것이
• 좋아요 = is good

So:

• 치실을 쓰는 것이 좋아요
• contracted to 치실을 쓰는 게 좋아요

Natural translation:

• It’s good to use floss
• You should use floss

This structure is often used for advice:

• 일찍 자는 게 좋아요 = It’s good to sleep early.
• 물을 많이 마시는 게 좋아요 = It’s good to drink a lot of water.

Because 쓰는 게 좋아요 is talking about the general action/habit of using floss, not a completed past action.

• 쓰는 게 좋아요 = using it is good / it’s good to use it
• 쓴 게 좋아요 would sound like the one that was used is good, which is not the intended meaning

When Korean turns a verb into a noun-like phrase meaning doing X, it often uses the present modifier form:

• 먹는 것 = eating / the thing you eat
• 운동하는 것 = exercising
• 치실을 쓰는 것 = using floss

So 쓰는 게 좋아요 is the correct form for advice about an action.

It is a gentle recommendation.

• 치실을 쓰는 게 좋아요 = It’s good to use floss / You should use floss

This sounds softer than:

• 치실을 써야 해요 = You have to / should use floss
• 치실을 쓰세요 = Please use floss

So the sentence gives advice in a polite, natural, non-pushy way.

The subject is omitted because Korean often leaves out information that is obvious from context.

Here, the implied subject is usually:

• you
• or people in general

So the full idea is something like:

• If you want to prevent cavities, it’s good to use floss at night too.

Korean does this all the time. If the listener can easily infer who the sentence is about, the subject is often left unstated.