muneul yeolnikka gangajiga nawasseoyo.

In this sentence, -니까 most naturally means when or once:

• 문을 열니까 = when I opened the door
• 강아지가 나왔어요 = a puppy came out

So the sentence is describing what happened right after the first action.

No. -니까 can mean either:

• because / since
• when / after doing

Here, the natural meaning is the when/after meaning, because the sentence describes a sequence of events:

1. the door was opened
2. then a puppy came out

So although -니까 often gets introduced as because, in real Korean it is also very common for when/once in this kind of sentence.

Because 문 is the object of the verb 열다 (to open).

• 문 = door
• 을/를 = object particle
• 열다 = to open

So:

• 문을 열다 = to open the door

The thing being opened gets 을/를.

Because 강아지 is the subject of 나오다 (to come out).

• 강아지가 나왔어요 = the puppy came out

Here, the puppy is the one doing the action of coming out, so it takes 이/가, not 을/를.

Also, 이/가 often feels natural when introducing or noticing something new. The speaker opens the door and then notices: a puppy came out.

나왔어요 is the polite past form of 나오다 (to come out / to appear).

Breakdown:

• 나오다 = to come out
• past marker: -았- / -었-
• polite ending: -어요

So:

• 나오다 → 나왔어요

Meaning: came out

In natural English, this could also be translated as:

• came out
• came running out
• appeared

depending on context.

This is about direction and perspective.

• 나오다 = to come out
• 나가다 = to go out

Korean often chooses between these based on the speaker’s point of view.

In this sentence, the puppy is coming out from the door/inside area into the speaker’s view, so 나오다 is natural.

If you used 나가다, it would sound more like the puppy is going out away from some reference point.

Because Korean often leaves out subjects when they are obvious from context.

In English, you usually need to say:

• When I opened the door...

But in Korean, if it is clear that the speaker is the one who opened it, you can simply say:

• 문을 열니까...

The listener naturally understands the missing subject.

Yes, it can.

A sentence like 문을 열니까 강아지가 나왔어요 often has a discovered result feeling:

• I opened the door,
• and then I found/realized that a puppy came out.

So it can sound a little like:

• When I opened the door, a puppy came out!
• I opened the door, and out came a puppy.

That slight sense of noticing or unexpected result is very common with this structure.

강아지 literally often means puppy, but in everyday Korean it can also refer to a small dog or be a cute, affectionate way to say dog.

Compare:

• 강아지 = puppy / doggy / little dog
• 개 = dog

개 is the basic word, but 강아지 sounds softer and cuter. So if someone is talking about a pet in a warm way, 강아지 is very common.

Because this is the standard polite everyday speech style.

• 나왔어요 = polite
• 나왔어 = casual/informal
• 나왔습니다 = formal

So this sentence is polite but natural for normal conversation.

Yes. A very common alternative would be:

• 문을 열었더니 강아지가 나왔어요.

This also means something like:

• When I opened the door, a puppy came out.

The nuance of -었더니 / -더니 is often very strong for:

• doing something
• then discovering or witnessing a result

So many learners feel that -었더니 sounds especially natural in this kind of I did X, and then I found Y situation.

Meanwhile, -니까 is broader, since it can also mean because in other contexts.

In standard conjugation, many learners are taught 여니까 from 열다, because the final ㄹ often drops before endings beginning with ㄴ.

So the form you will commonly learn is:

• 문을 여니까 강아지가 나왔어요.

If you encounter 열니까, the meaning is the same in context, but 여니까 is the safer standard form to remember for study.

Sometimes, yes, but when I opened the door or once I opened the door is usually better.

As I opened the door in English can sound like both actions happened at exactly the same time.
The Korean here usually gives more of a first action → resulting event feeling:

• I opened the door,
• then a puppy came out.

So when I opened the door is usually the most natural translation.