deungrokharyeomyeon magam jeone onraineuro sincheonghaeya hae.

-려면 attaches to a verb stem to express “if one intends to” or “in order to.”
• 등록하려면 = “if you want to register” or “to register.”
• Pattern: Verb stem + -려면 + [condition/result]
• Example: 공부하려면 (if you want to study), 일하려면 (in order to work)

Korean generally follows a time/place/manner + object + verb order.
• 마감 전에 (time) – “before the deadline”
• 온라인으로 (manner) – “online”
• 신청해야 해 (verb phrase) – “you must apply”
You can swap 마감 전에 and 온라인으로, but the verb phrase must stay last.

마감 means “deadline” or “closing.”
• Adding the particle -에 marks a point in time: 마감에 = “at the deadline.”
• With 전에 (“before”), it becomes 마감 전에 = “before the deadline.”

-기 전에 is a grammatical structure meaning “before doing [verb].”
• Here, 마감 is a noun (“deadline”), so we use 전에 directly after it to get “before the deadline.”
• If you had a verb, e.g. 수업이 끝나기 전에, it would mean “before the class ends.”

Both convey “apply online,” but:
• 온라인으로 신청하다 uses the adverbial particle -으로, emphasizing the means (“by/through online”).
• 온라인 신청하다 is common in spoken Korean; it treats 온라인 as a modifier but is slightly less formal.

신청해야 해 expresses obligation or necessity (“must apply”).
• 신청한다 is a simple statement (“you apply”).
• 신청할 수 있다 means “can apply” (possibility), not “must.”
If you want to soften obligation, you could say 신청해야 돼요 or more formally 신청해야 합니다.

Yes. Change 해야 해 to a more polite/formal ending:
• Polite: 등록하려면 마감 전에 온라인으로 신청해야 돼요.
• Formal: 등록하려면 마감 전에 온라인으로 신청해야 합니다.