Learn Hardcore Korean: hakgi deungrogeun magam jeone wanryohaja. - Let’s complete semester registration before the deadline.

In Korean, 은/는 is the topic marker, while 을/를 is the object marker. By saying 학기 등록은, the speaker is setting “semester registration” as the topic of the sentence, signaling “as for semester registration…” If you used 학기 등록을, you’d be directly marking it as the object of 완료하다 (“let’s complete semester registration…”), which is also possible but less topical. The nuance with 은/는 is more like “Regarding semester registration, let’s…”

What does 마감 전에 mean, and how is it constructed?

마감 전에 breaks down into:

마감 (Sino-Korean noun meaning “deadline” or “closing”)
전에 (from 전(前)
, “before”)

Together 마감 전에 literally means “before the deadline.” Whenever you want to say “before X” in Korean, you can attach 전에 to the noun.

Could I say 마감 전까지 완료하자 instead of 마감 전에 완료하자? What’s the difference?

Yes, you can say 마감 전까지 완료하자, but:

전에 means simply “before.”
전까지 means “up until” or “by.”

Using 전까지 adds a sense of “not later than that point,” often with a deadline urgency:
• 마감 전에 완료하자 – “Let’s finish before the deadline.”
• 마감 전까지 완료하자 – “Let’s finish by (no later than) the deadline.”

What is the ending -자 in 완료하자, and how formal is it?

The -자 ending is the “let’s” or suggestion form in Korean used in casual or polite-casual speech (banmal). It attaches to the verb stem:
• 완료하다 → stem 완료하- → 완료하자 (“let’s complete”)
This form is appropriate among friends, colleagues of the same rank, or when the speaker deliberately chooses a casual tone. For more formal settings, you’d say 완료합시다.

Why is there no subject like 우리 (“we”) in the sentence?

Korean often drops pronouns when the subject or object is obvious from context. Here, 우리 (“we”) is implied in the suggestion 완료하자. Native speakers omit 우리 because it’s understood:
• (우리) 학기 등록은 마감 전에 완료하자.
Both versions are correct, but the shorter form feels more natural in everyday speech.

What’s the difference between 완료하다 and 끝내다? Could I say 끝내자 instead?

Both mean “to finish,” but with slight nuance:

완료하다 (Sino-Korean) often sounds more formal or technical, emphasizing that a process or task has been completed in its entirety.
끝내다 (native Korean) is more general and conversational, meaning “to end/finish.”

You could say 끝내자:
• 학기 등록은 마감 전에 끝내자.
That’s perfectly natural, slightly less formal than 완료하자.

Can I replace 전에 with 후에? What would the meaning be?

No—전에 means “before,” and 후에 means “after.” They are opposites:
• 마감 전에 완료하자. = “Let’s complete it before the deadline.”
• 마감 후에 완료하자. = “Let’s complete it after the deadline,” which usually makes no sense for a deadline-based task. Ensure you choose 전에 when you need “before,” and 후에 for “after.”