kosmuri nageodeun maseukeureul sseo.

콧물 means nasal mucus or snot. In natural English, you would usually translate it as runny nose when talking about symptoms.

It is a set Korean noun:

• 코 = nose
• 물 = water / liquid

So it literally has the sense of nose liquid, though learners should just memorize 콧물 as the normal word.

-이 is the subject marker.

So:

• 콧물 = nasal mucus / runny nose
• 콧물이 = the mucus / the runny nose as the subject of the verb

In 콧물이 나다, the thing that is coming out / occurring is 콧물, so it takes the subject marker -이.

Korean often describes symptoms as something that comes out, appears, or occurs, rather than using have.

So:

• 콧물이 나다 = literally nasal mucus comes out
• natural English = to have a runny nose

This is just the normal Korean way to express that symptom.

Here, -거든 means if or when.

So 콧물이 나거든 마스크를 써 means:

• If you have a runny nose, wear a mask
• or When you have a runny nose, wear a mask

In this kind of sentence, -거든 often introduces a situation, and the second clause gives:

• advice
• an instruction
• a suggestion
• a command

It sounds very natural in spoken Korean.

Both can mean if/when, and in this sentence both work:

• 콧물이 나거든 마스크를 써
• 콧물이 나면 마스크를 써

The difference is mostly nuance:

• -(으)면 is more neutral and broad
• -거든 is more conversational and often sets up what to do in that situation

So -거든 here feels a bit like:

• if that happens, then...
• in that case...

The subject you is omitted because Korean often leaves out subjects that are obvious from context.

So the sentence naturally means:

• If you have a runny nose, wear a mask

Depending on context, it could also refer to someone in general, but most often it is understood as advice directed at you.

Yes. 써 comes from 쓰다.

Here, 쓰다 means to wear something on the head or face.

That is why Korean says:

• 모자를 쓰다 = wear a hat
• 안경을 쓰다 = wear glasses
• 마스크를 쓰다 = wear a mask

Also, 써 is the contracted form of 쓰어, which becomes 써.

Korean uses different verbs for wearing different kinds of items.

Common ones are:

• 쓰다 = wear on the head/face
• 입다 = wear clothes like shirts, jackets, pants
• 신다 = wear shoes/socks
• 끼다 = wear rings, gloves, glasses in some contexts
• 차다 = wear a watch, bracelet, etc.

A mask goes on the face, so 쓰다 is the normal verb:

• 마스크를 쓰다

No. 써 is casual speech.

It would be natural when speaking to:

• a friend
• a child
• someone younger
• someone you are close to

If you want to be polite, you could say:

• 콧물이 나거든 마스크를 쓰세요
• 콧물이 나면 마스크를 쓰세요

So the original sentence is informal/casual.

It can sound like either, depending on tone and context.

Because the second clause uses the imperative 써, it is grammatically a command, but in real life it often functions as practical advice:

• If you have a runny nose, wear a mask

So it may feel like:

• a friendly reminder
• health advice
• a parent telling a child what to do
• a direct instruction

The relationship between the speakers and the tone of voice matter a lot.