sincheongeun geumyoilkkaji haeya haeyo.

신청은 uses the topic particle 은, not the object particle 을/를.

In this sentence, 신청 is being presented as the topic: as for the application / regarding the application.
So the sentence feels like:

• As for the application, you have to do it by Friday.

If you said 신청을 금요일까지 해야 해요, that is also possible, but it sounds more like the application is the direct object of 해야 해요. In real Korean, 신청은 ... 해야 해요 is very natural because Korean often uses the topic particle to frame what is being talked about.

So:

• 신청은 금요일까지 해야 해요 = As for the application, it has to be done by Friday
• 신청을 금요일까지 해야 해요 = You have to submit/do the application by Friday

Both can work, but 은 gives a topic/contrast feeling.

Here 금요일까지 is best understood as by Friday.

The particle 까지 literally means something like until / up to, but in deadline situations it often translates naturally as by in English.

So in this sentence:

• 금요일까지 해야 해요 = must do it by Friday

This usually includes Friday as the final acceptable day.

Examples:

• 3시까지 와요 = Come by 3 o’clock
• 내일까지 끝내야 해요 = You have to finish it by tomorrow

So while 까지 often gets taught as until, the best translation depends on context.

해야 해요 expresses have to / must.

It comes from:

• 하다 = to do
• -아/어야 하다 = to have to do

Since 하다 becomes 해야 in this grammar pattern:

• 하다 → 해야 하다
• polite form: 해야 해요

So:

• 해요 = do / does
• 해야 해요 = have to do / must do

That is why 신청은 금요일까지 해야 해요 means not just do the application by Friday, but you must do the application by Friday.

Korean very often omits the subject when it is understood from context.

So 신청은 금요일까지 해야 해요 does not explicitly say you, but the listener usually understands it as something like:

• You have to apply by Friday
• The application must be done by Friday

Which one feels most natural depends on context.

Korean often leaves out words like:

• I
• you
• we
• sometimes even the object

If the situation already makes the meaning clear, Korean usually prefers omission.

Here, 신청 is functioning as a noun.

• 신청 = application / applying / registration
• 신청하다 = to apply / to register / to submit an application

In this sentence, the noun 신청 is used with 하다:

• 신청(을) 하다 = to apply

This is very common in Korean. Many action nouns combine with 하다 to make a verb.

Examples:

• 공부
  • 하다 = 공부하다 = to study
• 운동
  • 하다 = 운동하다 = to exercise
• 신청
  • 하다 = 신청하다 = to apply

So even though the sentence starts with 신청은, the full action is still basically 신청하다.

Yes, absolutely.
금요일까지 신청해야 해요 is very natural and probably the most straightforward version.

Compare:

• 신청은 금요일까지 해야 해요
• 금요일까지 신청해야 해요

The second version is more compact because 신청하다 stays together as one verb idea. It directly means:

• You have to apply by Friday

The original sentence with 신청은 slightly highlights the application as the topic, as if talking about that specific matter.

So the difference is mainly one of structure and emphasis, not core meaning.

It can.

The topic particle 은/는 often does one of two things:

1. marks the topic
2. adds a slight contrast

So 신청은 금요일까지 해야 해요 could simply mean:

• As for the application, you have to do it by Friday

But depending on context, it could also imply contrast, like:

• The application, at least, has to be done by Friday
• The application has to be done by Friday, though maybe something else can wait

This contrast is often subtle and depends on the situation. If there is no special contrast in context, learners should mainly understand 은 here as a topic marker.

해야 해요 is polite informal, often called the -요 form.

It is appropriate in many everyday situations:

• speaking politely to someone you do not know well
• customer service
• normal conversation
• workplace situations, depending on hierarchy

Compare:

• 해야 해 = casual
• 해야 해요 = polite
• 해야 합니다 = formal

So this sentence is polite, but not extremely formal.

It can do either, depending on context.

신청은 금요일까지 해야 해요 can mean:

• a direct instruction: You need to apply by Friday
• a rule or requirement: Applications must be submitted by Friday

Korean often leaves that distinction a little flexible. The context tells you whether the speaker is:

• giving advice,
• giving an order,
• explaining a deadline,
• or stating a general requirement.

So the grammar itself allows both interpretations.

Usually yes.

With 금요일까지, the normal interpretation is that Friday is the last acceptable day. In other words:

• Thursday or earlier = okay
• Friday = still okay
• after Friday = too late

This is the same way by Friday usually works in English.

If exact timing matters, Korean may add more detail, such as:

• 금요일 오후 6시까지 = by Friday at 6 PM
• 금요일 안으로 = within Friday / before Friday ends

But on its own, 금요일까지 normally includes Friday.