chinguga oneureun sigani eobseoseo mos ondaeyo.

온대요 is a reported-speech form. It means the speaker is passing along what they heard, rather than stating it as their own direct observation.

In full, it comes from something like:

• 못 온다고 해요 → 못 온대요

So 친구가 오늘은 시간이 없어서 못 온대요 means something like:

• My friend says they can’t come today because they don’t have time
• or I heard my friend can’t come today because they don’t have time

This gives the sentence a softer, more conversational feel than a plain direct statement.

못 오다 means cannot come or be unable to come.
안 오다 means do not come or are not coming.

Here, the reason is 시간이 없어서 — because they don’t have time. That suggests inability or lack of opportunity, so 못 오다 fits naturally.

• 못 와요 = can’t come
• 안 와요 = doesn’t come / isn’t coming

So this sentence is saying the friend is unable to come, not simply choosing not to.

없어서 is 없다 + -어서.

• 없다 = to not exist / to not have
• -어서 = because, so

So 시간이 없어서 means:

• because there is no time
• more naturally, because they don’t have time

In this sentence, it gives the reason for 못 온대요:

• 시간이 없어서 못 온대요
• They say they can’t come because they don’t have time

With 있다 and 없다, Korean usually marks the thing that exists or does not exist with 이/가, not 을/를.

So:

• 시간이 있다 = there is time / to have time
• 시간이 없다 = there is no time / to not have time

Even though English says have time, Korean treats it more like time exists / does not exist.

That is why 시간이 없어서 is correct.

The -은/는 here marks today as a topic and often adds a slight sense of contrast.

So 오늘은 can feel like:

• as for today
• today, at least
• today though

This often implies something like:

• maybe on another day the friend could come
• but today, they can’t

If you just said 오늘, it would simply mean today without that extra topical or contrastive nuance.

Both are possible in different contexts, but they give slightly different nuances.

• 친구가 marks friend as the subject and presents it as new or specific information.
• 친구는 would make friend the topic, often implying contrast or that the friend is already established in the conversation.

So:

• 친구가 오늘은 시간이 없어서 못 온대요
  = My friend can’t come today, I hear

• 친구는 오늘은 시간이 없어서 못 온대요
  = As for my friend, they can’t come today because they don’t have time

The version with 가 sounds very natural when simply reporting the situation.

Yes, in a way — but they belong to different parts of the sentence.

The sentence has a larger clause and a reason clause inside it:

• 친구가 ... 못 온대요
  = the friend can’t come, they say
• 오늘은 시간이 없어서
  = because today there is no time

So:

• 친구가 is the subject of 못 온대요
• 시간이 is the subject of 없어서

This is very normal in Korean. It is not a mistake or an unusual structure.

This sentence is in polite spoken Korean.

The final -요 makes it polite:

• 온대요

At the same time, -대요 is conversational and commonly used in speech.

Compare:

• 못 온대요 = polite, conversational, reported speech
• 못 온다고 해요 = also polite, a little fuller and slightly more explicit
• 못 온대 = casual / non-polite

So this sentence sounds natural in everyday polite conversation.

Yes, you can, but the meaning changes slightly.

• 친구가 오늘은 시간이 없어서 못 온대요
  = My friend says / I heard my friend can’t come today because they don’t have time

• 친구가 오늘 시간이 없어서 못 와요
  = My friend can’t come today because they don’t have time

The second one sounds like your own statement.
The original sentence sounds like you are relaying what the friend said.

Also, 오늘은 has a bit more contrast than plain 오늘.

Yes. Literally, 시간이 없다 is time does not exist or there is no time. But in natural English, it usually means:

• to not have time
• to be short on time

So in this sentence:

• 오늘은 시간이 없어서
  = because they don’t have time today

This is one of those very common Korean patterns where the literal structure and the natural English translation are a little different.

It can cover both, depending on context, but the main idea is reported information.

With -대요, the speaker is not presenting the information as a plain direct fact. They are passing it on:

• maybe the friend said it directly
• maybe someone else told the speaker
• maybe the speaker is relaying what they heard

So the sentence has a sense of:

• Apparently, my friend can’t come today because they don’t have time
• My friend says they can’t come today because they don’t have time

That reported feel is one of the most important things in this sentence.