Learn Hardcore Korean: hakgi deungrogeun geumyoilkkaji haeya haeyo. - You have to complete semester registration by Friday.

In Korean, subjects are often omitted when they’re clear from context. Here there is no explicit I or you—it’s a general instruction. Usually the implied subject is you (e.g. students) if a professor is talking, or we if a team is reminding itself. Without extra context, it simply means “(You) have to register for the semester by Friday.”

Why does 학기 등록 take the topic marker -은 instead of the object marker -을? What’s the nuance difference?

Korean particles mark the role of nouns.

-은 is the topic marker, signaling “as for semester registration …,” often highlighting or contrasting it.
-을 is the object marker, marking a direct object of the verb.

Both are grammatically possible:
• 학기 등록은 금요일까지 해야 해요 (“As for semester registration, it must be done by Friday.”)
• 학기 등록을 금요일까지 해야 해요 (“You must register for the semester by Friday.”)

Using 은 gives a slightly more “topic-comment” feel, whereas 을 is more straightforwardly “action-on-object.”

What does 까지 mean in 금요일까지, and how is it different from 전에 or just 금요일에?

까지 literally means “up to (and including)” a point.

금요일까지 = “until Friday” or “by Friday” (you can do it on Friday itself).

By contrast:

금요일 전에 = “before Friday” (excluding Friday itself).
금요일에 = “on Friday” (exactly that day, not a deadline).

What happens if we replace 까지 with 전에 or omit it entirely?

• 금요일 전에 해야 해요 ⇒ “You have to do it before Friday” (you cannot wait until Friday).
• 금요일에 해야 해요 ⇒ “You have to do it on Friday” (the action happens that day, not a range).
• Omitting any marker (금요일 해야 해요) is ungrammatical—Korean needs a particle to show the time relation.

What is the grammar behind 해야 해요? What does the pattern -아/어야 하다 express?

The construction 동사 stem + ‑아/어야 하다 expresses obligation or necessity (“have to/ must”).

Take the verb 하다 → stem 하
Add -아야 → 해야
Add 하다 and conjugate politely → 해야 해요

So 해야 해요 = “you/one has to do (it).”

How does 해야 해요 differ from 해야 돼요 and 해야 합니다?

• 해야 해요 (from 해야 하다) often feels like an internal or speaker-based obligation.
• 해야 돼요 (from 해야 되다) can feel more like an external rule or requirement.
In practice, they are nearly interchangeable in conversation.

Formality levels:

해야 해 / 해야 돼 = informal/plain
해야 해요 / 해야 돼요 = polite/informal
해야 합니다 = polite/formal (used in announcements, official contexts)

Is the word order fixed? Could I say 금요일까지 학기 등록은 해야 해요?

Korean is relatively flexible about placing time expressions and topics before the verb. Both are natural:
• 학기 등록은 금요일까지 해야 해요
• 금요일까지 학기 등록은 해야 해요

Putting 금요일까지 first shifts emphasis onto the deadline. However, the verb 해야 해요 must stay at the end; you wouldn’t say 금요일까지 해야 해요 학기 등록은.

What’s the difference between 등록 and 신청 in Korean? Why use 등록 here?

• 신청(하다) = “to apply for” or “to sign up.” Often used for applying to programs or selecting courses (수강 신청).
• 등록(하다) = “to register/enroll” in a more official or final sense (e.g. paying tuition, finalizing your semester).

Here 학기 등록 refers to the formal enrollment procedure for the semester, so 등록 is the appropriate term.