yeopseo-e jusoreul sseugo upyoreul butin da-eume baro uchetonge neoheosseo.

Here, 에 marks the place/surface where something is written. So 엽서에 주소를 쓰다 means to write an address on a postcard.

This is different from 엽서를 쓰다, which usually means to write a postcard in the sense of composing the message/content.

So:

• 엽서에 주소를 쓰다 = write the address on the postcard
• 엽서를 쓰다 = write a postcard

Because each one is the object of a different verb:

• 주소를 쓰고 = write the address
• 우표를 붙인 = attach the stamp

Korean often leaves out information that is obvious from context. In 우표를 붙이다, the thing you attach the stamp to is not repeated, because it is clearly the postcard.

So the full idea is something like:

• write the address
• attach the stamp to the postcard

-고 connects verbs. In this sentence, it links actions in sequence:

• 주소를 쓰고 = write the address, and then...
• 우표를 붙인 다음에 = after attaching the stamp...

Often -고 can be translated as and, but in action sequences it often feels like and then.

It does not strongly emphasize completion by itself; it just links actions naturally.

다음에 is used in the grammar pattern V-(으)ㄴ 다음에, which means after doing V.

So:

• 붙이다 → 붙인 다음에
• literally: after having attached

This is a very common pattern:

• 먹은 다음에 = after eating
• 간 다음에 = after going
• 읽은 다음에 = after reading

So 우표를 붙인 다음에 means after attaching the stamp.

You could also say:

• 우표를 붙이고 나서
• 우표를 붙인 후에

These are similar, though the nuance can vary slightly.

The object is omitted because it is obvious from context. The thing being put into the mailbox is the postcard.

So the full meaning is:

• (엽서를) 우체통에 넣었어
• I put the postcard into the mailbox

This kind of omission is extremely common in Korean when the listener can easily figure out what is meant.

With 넣다 (to put in / insert), Korean often marks the destination/location with 에.

So:

• 우체통에 넣다 = put it into the mailbox
• 가방에 넣다 = put it into a bag
• 상자에 넣다 = put it into a box

The object can also be stated if needed:

• 엽서를 우체통에 넣었어 = I put the postcard into the mailbox

So 에 here marks where it was put.

Yes, 다음에 can mean next in some contexts, but in this grammar pattern it means after.

In V-(으)ㄴ 다음에, it works like:

• after doing V

So here:

• 붙인 다음에 = after attaching

This is not the same as using 다음에 by itself to mean next time or later.

Compare:

• 다음에 봐 = see you next time / see you later
• 숙제한 다음에 놀았어 = I played after doing my homework

바로 means right away, immediately, or directly.

In this sentence, it emphasizes that the speaker put the postcard into the mailbox without delay after preparing it.

So it gives the sense of:

• wrote the address
• attached the stamp
• then immediately mailed it

Without 바로, the sentence would still be natural, but the sense of immediacy would be weaker.

넣었어 is the casual/informal style. It would be used with friends, family, or someone you speak casually with.

The polite version is:

• 넣었어요

So:

• 넣었어 = casual
• 넣었어요 = polite

The basic meaning is the same; only the speech level changes.

Korean often omits the subject when it is clear from context.

So even though the sentence does not explicitly say I, the listener understands that the speaker is talking about their own action:

• (나는) 엽서에 주소를 쓰고... 넣었어

In English, you usually need the subject, but in Korean it is very often left out when unnecessary.

Yes. The grammar makes the sequence very clear:

1. 엽서에 주소를 쓰고 = write the address on the postcard
2. 우표를 붙인 다음에 = after attaching the stamp
3. 바로 우체통에 넣었어 = immediately put it into the mailbox

So the sentence is not just listing actions randomly. It shows a clear real-world order.

Yes, that would be very natural.

For example:

• 엽서에 주소를 쓰고 우표를 붙이고 나서 바로 우체통에 넣었어.

This also means I wrote the address on the postcard, attached the stamp, and then immediately put it in the mailbox.

Both are common:

• V-(으)ㄴ 다음에
• V-고 나서

They both mean after doing V, though -고 나서 can sometimes feel a bit more conversational.

• 붙인 comes from 붙이다 = to attach, stick, paste
• 넣었어 comes from 넣다 = to put in

More specifically:

• 붙인 is the adnominal form used before 다음에
• 넣었어 is the past casual form of 넣다

So:

• 붙이다 → 붙인 다음에
• 넣다 → 넣었어

This kind of form change is very common in Korean grammar.