yeori nageodeun jibeseo swieo.

Literally, 열 means heat/fever, and 나다 means to come out / to occur / to arise. So 열이 나다 is literally something like a fever comes up or heat occurs.

In natural English, it means to have a fever.

Also, 열이 uses the subject particle 이 because 열 is what is occurring.

Because 나다 is an intransitive verb here. That means it describes something happening on its own, rather than something acting on an object.

So:

• 열이 나다 = a fever occurs / one gets a fever
• not 열을 나다

The particle 이/가 marks what appears or happens.

Here, -거든 means if or if it happens that.

So 열이 나거든 집에서 쉬어 means:

• If you have a fever, rest at home.

This ending often gives a condition for the next clause, especially in spoken Korean. It can feel a little like:

• if
• if that happens
• if that’s the case

It comes from:

• dictionary form: 나다
• verb stem: 나-
• conditional ending: -거든

So:

나다 → 나 + 거든 → 나거든

Since the stem ends in a vowel, -거든 attaches directly.

They are similar, because both can mean if, but they are not always exactly the same.

For this sentence, 열이 나면 집에서 쉬어 would also be natural and means nearly the same thing.

A useful difference:

• -(으)면 is the more neutral, general if/when
• -거든 is common in conversation and often sounds a bit more like if that happens, then...

In advice or instructions, -거든 can sound very natural:

• 아프거든 병원에 가.
• If you're sick, go to the hospital.

Because 쉬어 is the informal present/imperative-style form of 쉬다.

Here it is being used as a casual command or advice:

• 쉬어 = rest
• 쉬다 = dictionary form, to rest

So the sentence is giving direct advice in casual speech.

쉬어 is casual speech, used with:

• close friends
• younger people
• children
• someone you speak informally to

쉬세요 is polite:

• 열이 나거든 집에서 쉬세요.
• If you have a fever, please rest at home.

So the original sentence is in informal/plain casual style.

Because 쉬다 describes an action happening at a location, so 에서 is used.

• 집에서 쉬어 = rest at home
• 에서 marks the place where the action happens

By contrast, 집에 usually marks a destination or location of existence:

• 집에 가다 = go home
• 집에 있다 = be at home

So with 쉬다, 집에서 is the natural choice.

Yes. Korean often omits subjects when they are clear from context.

In this sentence, the understood subject is usually you:

• (네가) 열이 나거든 집에서 쉬어.
• If you have a fever, rest at home.

But Korean does not need to say you explicitly unless there is a reason to emphasize it.

It can feel like any of these depending on tone and context, but most naturally it sounds like advice or a casual instruction.

Examples:

• a parent to a child
• a friend to a friend
• someone looking after another person

Because it uses 쉬어, it is direct, but in many contexts it feels more like You should rest at home than a harsh command.

Yes. That is one of the common uses learners see.

A pattern like this is very common:

• X-거든 Y해
• If X happens, do Y

Examples:

• 시간 있거든 전화해.

• If you have time, call me.

• 배고프거든 먼저 먹어.
• If you're hungry, eat first.

So 열이 나거든 집에서 쉬어 fits this pattern very naturally.

It can, but 열이 나다 is the more common and natural expression for to have a fever.

Compare:

• 열이 나다 = the standard expression for having a fever
• 열이 있다 = literally there is fever/heat, which can be understood, but is often less idiomatic in everyday speech for a person’s fever

So if you are talking about someone having a fever, 열이 나다 is usually the best choice.

Yes. Korean commonly puts the condition first, then the result/advice after it.

So the structure is:

• condition: 열이 나거든
• main action/advice: 집에서 쉬어

This is a very normal Korean way to organize the sentence:

• If you have a fever, rest at home.

It sets up the situation first, then tells you what to do.

It is very natural in spoken Korean, especially in casual conversation.

It sounds like something a parent, friend, or acquaintance might say. If you want to make it more polite, you can simply change the ending:

• 열이 나거든 집에서 쉬어. — casual
• 열이 나거든 집에서 쉬어요. — polite casual
• 열이 나거든 집에서 쉬세요. — polite and more clearly advisory/request-like

So the original sentence itself is completely natural; it is just in an informal speech level.