jumeonie chisireul neoheo dumyeon jeomsimeul meokgo naseodo baro sseul su isseoyo.

Korean often leaves out the subject when it is obvious from context.

In this sentence, the omitted subject is something like you, one, or we in a general-advice sense. English usually wants a subject, but Korean does not need to say it if the listener can easily understand it.

So this sentence feels like:

• If you keep dental floss in your pocket...
• If one puts dental floss in one’s pocket...

The Korean sentence is perfectly natural without saying the subject explicitly.

Here, 에 marks the place something is put into.

With verbs like 넣다 (to put in), 에 often shows the destination or location:

• 가방에 넣다 = put into a bag
• 서랍에 넣다 = put into a drawer
• 주머니에 넣다 = put into a pocket

Using 에서 would not work here, because 에서 usually marks the place where an action happens, not the destination something is put into.

So 주머니에 치실을 넣다 means to put dental floss into a pocket.

넣어 두면 is made from:

• 넣다 = to put in
• 넣어 두다 = to put in and leave it there / keep it there in advance
• 넣어 두면 = if you put it in and leave it there / if you keep it there

The grammar -아/어 두다 often means:

1. doing something in advance for later convenience, or
2. leaving something in a certain state

So 치실을 넣어 두면 is not just if you put floss in your pocket. It has the nuance of if you keep some floss in your pocket beforehand, so it is ready when you need it.

This is a very common question.

• 넣으면 = if you put it in
• 넣어 두면 = if you put it in and keep it there / if you leave it there ready for later

So 넣으면 is more neutral and just describes the action. 넣어 두면 adds the idea of preparation or keeping something available.

Compare:

• 주머니에 치실을 넣으면... = If you put floss in your pocket...
• 주머니에 치실을 넣어 두면... = If you keep floss in your pocket / If you put floss in your pocket ahead of time...

In this sentence, 넣어 두면 sounds more natural because the whole point is being able to use it later.

-고 나서 means after doing something.

So:

• 먹고 나서 = after eating
• 점심을 먹고 나서 = after eating lunch

It shows sequence: first you eat lunch, then something else happens.

A helpful way to think about it is:

• A-고 나서 B = do A, and then afterward do B

So here, it sets up the later action of using the floss.

도 adds the sense of also, even, or still depending on context.

Here, 점심을 먹고 나서도 suggests something like:

• even after eating lunch
• after lunch too
• still after eating lunch

It emphasizes that the floss is useful at that later time as well.

Without 도, 점심을 먹고 나서 would simply mean after eating lunch. With 도, the sentence sounds a little more like you can use it right away even after lunch.

Yes, 을 can often be omitted in casual speech.

So both are possible:

• 점심을 먹고 나서도
• 점심 먹고 나서도

The version with 을 is a little more explicit and grammatically complete. The version without it is very common in everyday conversation.

A learner should recognize both, but using 점심을 먹고 is perfectly fine and clear.

쓸 수 있어요 comes from:

• 쓰다 = to use
• -(으)ㄹ 수 있다 = can / be able to

So:

• 쓰다 → 쓸 수 있다 = can use
• 쓸 수 있어요 = can use (polite style)

This is a very common Korean pattern:

• 먹을 수 있어요 = can eat
• 갈 수 있어요 = can go
• 볼 수 있어요 = can see/watch
• 쓸 수 있어요 = can use

One thing that can confuse learners is that 쓰다 has several meanings, such as to write, to wear, and to use. Here, because the object is 치실, it clearly means to use.

Both can mean to use, but 쓰다 is usually more natural and common in everyday speech.

• 치실을 쓰다 = use dental floss
• 치실을 사용할 수 있어요 = can use dental floss

The second one is not wrong, but it sounds a bit more formal or less conversational.

Korean often prefers 쓰다 for ordinary objects and tools:

• 이 펜 써도 돼요? = Can I use this pen?
• 카드 쓸 수 있어요? = Can I use a card?

So 바로 쓸 수 있어요 sounds very natural here.

바로 means right away, immediately, or directly.

It emphasizes that because the floss is already in your pocket, you do not have to go look for it first. You can use it at once.

So the nuance is not just you can use it, but you can use it immediately.

In this sentence, 바로 naturally goes before 쓸 수 있어요 to describe how quickly the action can happen.

This sentence uses the standard polite -아요/어요 style.

So 쓸 수 있어요 is polite and natural for everyday conversation. It is appropriate in many normal situations.

Compare:

• 쓸 수 있어요 = polite everyday style
• 쓸 수 있습니다 = more formal
• 쓸 수 있어 = casual, non-polite

So the whole sentence sounds like normal polite spoken Korean.