seonsaengnimi naeil sueobeun onraineuro handaeyo.

A natural breakdown is:

• 선생님이 = the teacher + subject marker
• 내일 = tomorrow
• 수업은 = as for the class / the class (topic)
• 온라인으로 = online / in an online way or format
• 한대요 = says that they will do it / said that they’ll do it

So the sentence is basically:

The teacher says that tomorrow’s class will be online.

More literally, it is something like:

The teacher says, as for tomorrow’s class, they’ll do it online.

한대요 is a reported speech form. It comes from 한다고 해요.

• 하다 = to do
• 한다고 해요 = says that they do / says that they will do
• In everyday speech, 한다고 해요 is often shortened to 한대요

So 온라인으로 한대요 means:

• (they) say they’ll do it online
• or more naturally, (the teacher) says the class will be online

It shows that the speaker is reporting what someone said, not just stating the fact directly.

Here, 선생님이 marks the teacher as the subject of the reporting.

In other words, the teacher is the person who is saying the information.

You can think of the full idea as something like:

선생님이 [내일 수업은 온라인으로 해요]라고 해요
= The teacher says, “Tomorrow’s class is online / will be online.”

So 선생님이 is not the topic of the whole conversation so much as the source/speaker of the reported message.

If you used 선생님은, the nuance would shift more toward as for the teacher... rather than simply identifying the teacher as the one who said it.

The 은/는 here marks 수업 as the topic inside the reported message:

내일 수업은 온라인으로 한대요
= As for tomorrow’s class, it will be online

This has a very natural Korean feeling. It can also carry a slight sense of contrast, like:

• Tomorrow’s class, at least, is online
• or As for tomorrow’s class, it’s online

If you said 수업이, it would sound more like a plain subject statement. That may still be possible in some contexts, but 수업은 is very common and natural when introducing or highlighting the class as the topic.

In this sentence, -으로 shows the method, mode, or format.

So:

• 온라인 = online
• 온라인으로 하다 = to do it online / to hold it online / to conduct it online

This does not mean physical direction like toward online. Here, -으로 is more like:

• by
• in
• using
• in the form of

So 온라인으로 한대요 means the class will be conducted in an online format.

Because Korean often uses a present-style form for a planned or scheduled future event, especially when a future time word is already present.

Here, 내일 already tells you the event is in the future.

So Korean does not need an extra future marker to make the meaning clear.

This is very common:

• 내일 가요 = I’m going tomorrow
• 다음 주에 시험 봐요 = We have a test next week
• 내일 수업은 온라인으로 해요 = Tomorrow’s class is online / will be online

So the future meaning comes naturally from 내일 plus context.

Yes. In Korean, a time word often comes directly before a noun to describe it.

So:

• 내일 수업 = tomorrow class = tomorrow’s class
• 오늘 회의 = today meeting = today’s meeting
• 다음 주 시험 = next week test = next week’s test

So 내일 수업은 means as for tomorrow’s class.

Yes, in meaning, they are essentially the same here.

• 한다고 해요 = the full form
• 한대요 = a shortened, conversational form

So these are very close:

• 선생님이 내일 수업은 온라인으로 한다고 해요
• 선생님이 내일 수업은 온라인으로 한대요

The shorter form is very common in spoken Korean and casual writing.

Both can work depending on context.

Grammatically, 한대요 is closer to says that... / I hear that... in reported speech. But in natural English, it is often translated as:

• The teacher said tomorrow’s class will be online
• The teacher says tomorrow’s class will be online
• I heard that tomorrow’s class will be online, according to the teacher

English chooses tense more freely based on context, while Korean reported speech often focuses more on the fact that the speaker is passing along information.

So if you already got a message from the teacher, The teacher said... is often a very natural translation.

Yes, you may hear that too, but the nuance is a little different.

• 온라인으로 해요 = we do/hold/conduct it online
• 온라인이에요 = it is online

Both can be natural, but 온라인으로 하다 focuses on the way the class is conducted, while 온라인이다 treats online more like the class’s status or format.

In this sentence, 온라인으로 한대요 sounds very natural because Koreans often say that an event or class is done online.

So:

• 내일 수업은 온라인으로 한대요 = They say tomorrow’s class will be held online
• 내일 수업은 온라인이래요 = They say tomorrow’s class is online

Both are possible, but the original sentence is especially common and natural.