upyoga eobseumyeon soporeul bonaegi jeone uchegugeseo meonjeo saya hae.

Because 없다 works like to not exist / to not have, and the noun before it is usually marked with 이/가.

So:

• 우표가 없다 = there is no stamp / you don’t have a stamp

Korean is not treating 우표 as a direct object here, so 을/를 is not used.

You may also see 우표는 없으면, but that would add a contrastive feeling, like if, as for stamps, you don’t have any.

-으면 / -면 makes a condition: if.

So:

• 없다 = to not exist / to not have
• 없으면 = if there isn’t one / if you don’t have one

In this sentence, it sets up the situation for what comes next:

• If you don’t have a stamp, ...

A quick pattern:

• 비가 오면 = if it rains
• 시간이 없으면 = if you don’t have time
• 우표가 없으면 = if you don’t have a stamp

Because 전에 means before, and when you want to say before doing something, Korean often turns the verb into a noun-like form with -기.

So:

• 보내다 = to send
• 보내기 = sending
• 보내기 전에 = before sending

Literally, it is something like:

• before the act of sending the package

This is a very common pattern:

• 먹기 전에 = before eating
• 자기 전에 = before sleeping
• 공부하기 전에 = before studying

Because 전에 usually cannot attach directly to a bare verb stem in this kind of structure. You normally need the noun-forming -기:

• 보내기 전에 = before sending

So 보내 전에 is not the normal form here.

A useful rule of thumb:

• noun + 전에 → okay
  
  • 식사 전에 = before the meal
• verb + 기 전에 → okay
  
  • 먹기 전에 = before eating

They are related, but they do different jobs.

• 전에 = before; it sets the time relationship between two actions
• 먼저 = first; it emphasizes order or priority

In this sentence:

• 소포를 보내기 전에 = before sending the package
• 먼저 사야 해 = you have to buy it first

So 전에 tells you when the buying must happen, and 먼저 highlights that buying comes ahead of sending.

The sentence could work without 먼저, but 먼저 adds emphasis:

• 우표가 없으면 소포를 보내기 전에 우체국에서 사야 해.
  Still natural, just a little less emphatic.

Because 에서 marks the place where an action happens.

Here, the action is buying:

• 우체국에서 사야 해 = you have to buy it at the post office

Compare:

• 우체국에 가다 = go to the post office
  (에 marks destination)
• 우체국에서 사다 = buy at the post office
  (에서 marks location of action)

So in this sentence, the buying happens at the post office, which is why 에서 is used.

The object is omitted because it is already obvious from context: 우표.

So the full idea is:

• 우표를 먼저 사야 해 = you have to buy the stamp first

Korean very often leaves out words that are clear from the previous context. Since the sentence already started with 우표가 없으면, there is no need to repeat 우표를 again unless the speaker wants extra clarity or emphasis.

Both are possible:

• 우표가 없으면 ... 먼저 사야 해.
• 우표가 없으면 ... 우표를 먼저 사야 해.

The first one sounds more natural in everyday Korean because it avoids unnecessary repetition.

사야 해 comes from the grammar pattern -아/어야 하다, which expresses necessity: must / have to / should.

Breakdown:

• 사다 = to buy
• 사야 하다 = must buy / have to buy
• 사야 해 = casual spoken form of 사야 하다

So:

• 가야 해 = have to go
• 먹어야 해 = have to eat
• 사야 해 = have to buy

This does not mean simple future, like will buy. It specifically adds the meaning of necessity or obligation.

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

In English, you usually need to say:

• you have to buy it

In Korean, if it is clear who the sentence is about, the subject can be dropped:

• 사야 해 = have to buy it

In this sentence, the understood subject is something like:

• you
• one
• the person sending the package

Exactly who it refers to depends on context, but Korean does not need to state it explicitly every time.

Yes. 사야 해 is casual, informal speech.

A more polite version would be:

• 우표가 없으면 소포를 보내기 전에 우체국에서 먼저 사야 해요.

A more formal style could be:

• 우표가 없으면 소포를 보내기 전에 우체국에서 먼저 사야 합니다.

So the levels are roughly:

• 사야 해 = casual
• 사야 해요 = polite everyday
• 사야 합니다 = formal

By itself, 우표 does not clearly mark singular or plural. Korean nouns often do not change form for number.

So 우표 could mean:

• a stamp
• stamps

The exact meaning comes from context. In this sentence, English would often say a stamp, because that sounds natural, but Korean leaves it open unless the speaker specifically adds something like:

• 우표 한 장 = one stamp
• 우표들 = stamps (plural, though this is less necessary in many cases)

Yes. Korean word order is flexible as long as the grammar markers stay clear.

For example, these are all possible:

• 우표가 없으면 소포를 보내기 전에 우체국에서 먼저 사야 해.
• 우표가 없으면 먼저 우체국에서 사야 해.
• 우표가 없으면 우체국에서 먼저 사야 해.

The original sentence is nice because it clearly shows:

1. the condition: 우표가 없으면
2. the time relation: 소포를 보내기 전에
3. the place: 우체국에서
4. the priority: 먼저
5. the obligation: 사야 해

So the order is very natural and easy to understand, even though some rearrangement is possible.