naeil sueobi onraininji dasi hwaginhaeyo.

The sentence breaks down like this:

• 내일 = tomorrow
• 수업이 온라인인지 = whether the class is online
• 다시 확인해요 = check again / confirm again

So the structure is:

• [whether tomorrow’s class is online] + [check again]

A very common Korean pattern is to put the thing being checked, known, asked, or thought before the main verb.

-인지 means whether it is... or if it is...

Here:

• 온라인 = online
• 온라인이다 = to be online
• 온라인인지 = whether it is online

So 수업이 온라인인지 means whether the class is online.

This ending is often used when someone is checking, asking, wondering, or finding out something.

For example:

• 맞는지 확인해요 = I check whether it is correct.
• 어디인지 알아요 = I know where it is.

This is a very common learner question.

In 수업이 온라인인지, 수업 is the subject inside the embedded clause:

• 수업이 온라인이다 = the class is online

Then that whole clause becomes the thing being checked:

• [수업이 온라인인지] 확인해요 = check whether the class is online

So:

• 이/가 marks 수업 as the subject of 온라인이다
• the whole clause 수업이 온라인인지 functions as the object of 확인해요

That is why 수업을 온라인인지 would be wrong here.

Because this sentence is not directly saying The class is online or asking Is the class online?

Instead, it is talking about whether the class is online.

Compare these:

• 수업이 온라인이에요. = The class is online.
• 수업이 온라인인가요? = Is the class online?
• 수업이 온라인인지 확인해요. = I check whether the class is online.

So -인지 is the form you use when the idea becomes part of a larger sentence like check whether..., know whether..., ask whether..., and so on.

온라인 behaves like a noun in Korean, and it combines with the copula 이다.

So the underlying form is:

• 온라인이다 = to be online

When you attach the whether/if ending to a noun + 이다, you get:

• 온라인 + 인지 = 온라인인지

So this is not a random spelling. It comes from the noun pattern.

다시 means again.

So 다시 확인해요 means:

• check again
• confirm again

It implies that the speaker is checking one more time, maybe because they are not sure, or because plans may have changed.

Yes, 내일 수업 naturally means tomorrow’s class.

Korean often puts a time word directly before a noun without 의:

• 오늘 날씨 = today’s weather
• 내일 수업 = tomorrow’s class
• 어제 뉴스 = yesterday’s news

Using 의 is usually unnecessary here.
So 내일 수업 is the normal, natural way to say it.

Korean very often leaves out the subject when it is obvious from context.

So 확인해요 could mean:

• I check
• we check
• sometimes a general check

The exact subject depends on the situation. Korean does this much more often than English, so omitted subjects are something learners see all the time.

The dictionary form is 확인하다, which means to check or to confirm.

Conjugation:

• 확인하다 → 확인해요

This happens because 하다 verbs change like this in the polite -요 style:

• 공부하다 → 공부해요
• 전화하다 → 전화해요
• 확인하다 → 확인해요

So 확인해요 is just the present polite form of 확인하다.

It is the polite -요 style, which is very common in everyday Korean.

You could compare:

• 확인해 = casual
• 확인해요 = polite everyday
• 확인합니다 = more formal

So this sentence is polite, but not stiff or highly formal.

Yes.

Casual:

• 내일 수업이 온라인인지 다시 확인해.

Formal:

• 내일 수업이 온라인인지 다시 확인합니다.

The grammar stays the same; only the speech level changes.

Because Korean usually puts modifiers and subordinate clauses before the main verb.

English:

• I check whether tomorrow’s class is online.

Korean:

• Tomorrow class online-whether again check.

That may feel backwards to an English speaker, but it is very normal in Korean.
A useful habit is to read Korean sentences by waiting for the final verb, because the main action often comes at the end.

Yes, both can mean to check, but 확인하다 is usually the more standard and natural choice here.

• 확인하다 = confirm, verify, check
• 체크하다 = check, often more casual or English-like

For checking whether a class is online, 확인하다 sounds very natural.

So:

• 내일 수업이 온라인인지 다시 확인해요. = very natural

As written, it is a statement in the polite style:

• I check again whether tomorrow’s class is online
• or We check again..., depending on context

If you wanted a command, you would usually say:

• 내일 수업이 온라인인지 다시 확인하세요. = Please check again whether tomorrow’s class is online.

So 확인해요 is not the usual command form.

Yes. The same pattern works with many other words.

For example:

• 내일 수업이 취소됐는지 다시 확인해요. = I check again whether tomorrow’s class was canceled.
• 내일 수업이 있는지 다시 확인해요. = I check again whether there is class tomorrow.
• 내일 수업이 대면인지 다시 확인해요. = I check again whether tomorrow’s class is in person.

This shows that the key pattern is:

• A인지 / V는지 / adjective-form + 지
  • 확인하다

which is a very useful grammar pattern in Korean.