Learn Hardcore Korean: ibeon ju sueobeun modu onraineuro haeyo. - This week’s classes are all held online.

이번 주 literally means “this week.”
• 이번 = “this (coming/ongoing)”
• 주 = “week”
Together, 이번 주 specifies the current week, so the speaker is talking about classes taking place during the week right now.

Why is there the particle 은 after 수업 instead of 이/가?

은 is the topic marker, indicating what the sentence is about.
• 수업은 = “As for the classes…”
Using 은 places emphasis on the topic (the classes), rather than simply stating a subject. If you used 수업이, you’d be making a neutral statement: “The classes are all online.” With 수업은, it’s more like “Regarding the classes (this week), they are all online.”

What role does 모두 play in 수업은 모두 온라인으로 해요?

모두 means “all” or “everything.” Here it indicates that every single class this week is online. You’re basically saying, “All of the classes are done online.” Without 모두, you’d simply say “The classes are online,” which could imply most or some classes rather than every one.

Why do we use 온라인으로 instead of 온라인에 or 온라인에서?

-으로 here marks means or method:
• 온라인으로 해요 = “do it by/through online.”
If you said 온라인에 해요, it would sound like you’re placing the action inside a location called “online,” which isn’t natural. 온라인에서 해요 would be like “do it at the online place,” which is also awkward. The correct way to say “by online means” is 온라인으로.

What does 해요 mean in this context? It usually means “to do,” right?

Yes, 해요 = “do,” but when paired with 수업을, 수업을 하다 means “to conduct/hold a class” or “to take a class,” depending on context. Here it means “we hold (our) classes online.” In a student-centered context, you could also interpret it as “we have our classes online.”

Can we rephrase the sentence as 온라인 수업을 모두 해요?

You can say 온라인 수업을 모두 해요, but the nuance shifts slightly:
• 이번 주 온라인 수업을 모두 해요.
This still means “This week, we do all (our) classes online,” but it bundles 온라인 directly with 수업 (“online classes”), rather than focusing first on “classes” as the topic. Both are correct, though the original emphasizes 수업은 (“as for this week’s classes…”).

Is there an alternative to 모두 that’s more formal?

Yes, 전부 is a formal synonym of 모두. You could say:
• 이번 주 수업은 전부 온라인으로 해요.
It means exactly the same thing (“all”), but carries a slightly more formal tone.

If I want to drop 모두, will the meaning change?

You can drop 모두:
• 이번 주 수업은 온라인으로 해요.
Now it simply states “The classes are (being) online this week,” which most listeners will understand as “all of them” by default. 모두 adds clarity and emphasis that there are no exceptions.