nalssiga chuumyeon mokdorireul haeyo.

Both can be possible, but 날씨가 추우면 is the more neutral way to say if/when the weather is cold.

• 가 marks 날씨 as the subject of 춥다 (to be cold).
• 는 would add more of a topic or contrast feeling.

So:

• 날씨가 추우면 = if the weather is cold
• 날씨는 추우면 = less natural here unless you are contrasting the weather with something else

For a learner, it is safest to understand 날씨가 추우면 as the normal pattern.

추우면 means if it is cold or when it is cold.

It comes from the adjective 춥다 (to be cold).
The ending -으면 means if/when.

So the breakdown is:

• 춥다 = to be cold
• 추우면 = if/when it is cold

This form looks a little unusual because 춥다 is an irregular adjective. The ㅂ changes when certain endings are added:

• 춥다
• 추워요
• 추우면

So even though the dictionary form is 춥다, the conditional form becomes 추우면.

It can mean either if or when, depending on context.

In this sentence, 날씨가 추우면 목도리를 해요, it usually means something like:

• If the weather is cold, I wear a scarf
• When the weather is cold, I wear a scarf

In English, both are natural. The Korean -으면 often covers both ideas:

• a condition (if)
• a repeated situation or habit (when/whenever)

Here it sounds like a habitual action, so when it’s cold, I wear a scarf is often the most natural English interpretation.

Yes, 하다 usually means to do, but in Korean it is also used in some clothing/accessory expressions to mean to wear/put on.

So 목도리를 해요 means to wear a scarf.

This is a very natural Korean expression. With accessories and things worn on the body, Korean often uses verbs that do not match English word-for-word.

For scarves, you may also hear:

• 목도리를 해요 = wear a scarf
• 목도리를 매요 = tie/wear a scarf
• 목도리를 두르다 = wrap a scarf around oneself

Among these, 목도리를 해요 is common and conversational.

Because 목도리 is the object of 해요 in this expression.

• 목도리 = scarf
• 목도리를 = scarf + object marker

Even though English says wear a scarf, Korean uses the expression 목도리를 하다, so the scarf is treated as the object of 하다.

That is why you get:

• 목도리를 해요

and not just 목도리 해요 in careful standard Korean.

It is omitted because Korean often leaves out subjects when they are understood from context.

So this sentence literally looks like:

• If the weather is cold, wear a scarf

But in normal use, it usually means:

• I wear a scarf when it’s cold

Depending on context, it could also mean:

• people wear scarves when it’s cold
• you wear a scarf when it’s cold

Korean often does not state I/you/he/she unless needed.

해요 is the polite present-style form of 하다.

So it gives the sentence a polite, everyday tone:

• 해요 = polite, standard conversation style

This sentence is not talking only about right now. In context, present tense in Korean can also express:

• habits
• general truths
• things someone usually does

So 목도리를 해요 here means something like:

• I wear a scarf
• I usually wear a scarf

not necessarily I am putting on a scarf right now.

Usually it sounds like a general habit.

So the natural reading is:

• When it’s cold, I wear a scarf
• If the weather is cold, I wear a scarf

Because the main verb is in the present polite form (해요), it often describes what someone generally does in that situation.

If you wanted to make it more clearly about a future situation, Korean could use other context or wording. But as written, this sounds like a normal repeated habit.

No, 입어요 would sound wrong for a scarf.

Korean uses different verbs for different kinds of clothing/accessories:

• 입다 = wear clothes on the body, like shirts, coats, pants
• 쓰다 = wear on the head, like a hat
• 신다 = wear on the feet, like shoes
• 끼다 = wear/put on things like gloves, rings, glasses
• 하다 / 매다 / 두르다 = used for things like scarves or accessories in certain expressions

So for 목도리, 해요 is natural, while 입어요 is not.

Because 춥다 is the normal word for weather feeling cold.

• 춥다 = cold weather / feeling cold
• 차갑다 = physically cold to the touch, like cold water, cold metal, a cold hand

So:

• 날씨가 추워요 = the weather is cold
• 물이 차가워요 = the water is cold

Using 차갑다 for weather is usually not the standard choice in a basic sentence like this.