oneureun chuunikka changmuneul dadayo.

은 is the topic marker, so 오늘은 means something like as for today or today, ....

In this sentence, 오늘 is setting the scene or topic:

• 오늘은 = today / as for today

If you used 오늘이, that would mark today as the grammatical subject, which would sound less natural here.

So:

• 오늘은 추우니까... = Since it’s cold today...

The sentence is not really focusing on today as the subject; it is using today as the topic.

-니까 means because or since.

So:

• 추우니까 = because it is cold / since it is cold

In this sentence:

• 오늘은 추우니까 창문을 닫아요.
• Since it’s cold today, [I’m / let’s / please] close the window.

A very important point: -니까 often gives a reason that leads to a suggestion, command, or decision. That makes it very natural here.

These are completely different endings.

• 춥니까? = Is it cold? or the formal polite statement/question form of 춥다
• 추우니까 = because it is cold

So even though they look similar, they are built differently.

Also, 춥다 is a ㅂ-irregular adjective. When a vowel-starting ending is added, the ㅂ changes:

• 춥다
• 추워요
• 추우니까

So 추우니까 is the correct form for because it is cold.

This happens because 춥다 is a ㅂ-irregular word.

For many adjectives and verbs like this, when the next ending starts with a vowel, the final ㅂ changes to 우.

Pattern:

• 춥다 → 추워요
• 춥다 → 추우니까
• 덥다 → 더워요
• 덥다 → 더우니까

So:

• dictionary form: 춥다
• with -니까: 추우니까

This is just a conjugation rule you need to get used to.

Both can mean because, but they are not always interchangeable.

In this sentence, -니까 is especially natural because the second part can sound like:

• a decision
• a suggestion
• a command/request

That fits -니까 well.

Compare:

• 추우니까 창문을 닫아요.
  Since it’s cold, let’s/I’ll/please close the window.

• 추워서 창문을 닫아요.
  I close the window because it’s cold.

With -아서/어서, the second clause sounds more like a straightforward result.
With -니까, the reason often feels more explicit and can support commands or suggestions better.

A useful beginner rule:

• use -아서/어서 for natural cause/effect
• use -니까 when giving a reason for an action, suggestion, or command

을/를 is the object marker.

The verb 닫다 means to close, and the thing being closed is the object.

So:

• 창문 = window
• 창문을 = the window, as the object of close

In other words:

• 창문을 닫아요 = close the window

If you remove 을, the sentence may still be understood in casual speech, but 창문을 is the standard full form.

닫아요 is the polite present-style form of 닫다 meaning to close.

So 닫아요 can mean:

• I close it
• I’m closing it
• let’s close it
• please close it

Which one it means depends on context and intonation.

In Korean, this polite -아요/어요 ending is very flexible. It does not always correspond to just one English form.

So in this sentence, depending on context, it could feel like:

• I’m closing the window because it’s cold today.
• Let’s close the window because it’s cold today.
• Close the window because it’s cold today.

Yes, from an English point of view, the subject feels missing, but this is very normal in Korean.

Korean often omits subjects when they are obvious from context.

In this sentence:

• 오늘은 gives the topic
• 추우니까 gives the reason
• 창문을 닫아요 gives the action

The listener can infer the subject:

• I
• we
• you
  depending on the situation

Korean does this all the time, so you should not expect every sentence to include an explicit subject.

Yes, it can, depending on the situation.

In Korean, the polite -아요/어요 form can sometimes function like a soft instruction or suggestion, especially in conversation.

So:

• said by a teacher, parent, or coworker, it might sound like Please close the window.
• said by someone including themselves, it might sound like Let’s close the window.
• said while acting, it might mean I’m closing the window.

The grammar itself does not force only one interpretation. Context decides.

Korean often puts the reason first and the main action second.

So this structure is very natural:

• 오늘은 추우니까 창문을 닫아요.
• Since it’s cold today, [we/I/please] close the window.

This is a common pattern:

• reason + main action

You can also rearrange things in Korean, but this order is very standard and easy to understand.

For learners, it helps to think of the sentence as:

• 오늘은 = as for today
• 추우니까 = because it’s cold
• 창문을 닫아요 = close the window

By itself, 창문 does not clearly show singular or plural.

So 창문을 닫아요 could mean:

• close the window
• close the windows

Usually context tells you which one is meant. If Korean speakers need to make plurality clear, they can do so, but often they simply do not mark it.

In this sentence, English would usually translate it as the window, because that sounds most natural unless the context says otherwise.