annaebangsongeul deutgo naseoya seunggangjangi bakkwin geol arasseoyo.

A natural breakdown is:

• 안내방송을 = the announcement
  • object marker 을
• 듣고 나서야 = after listening
  • only then / not until then
• 승강장이 바뀐 걸 = that the platform had changed
• 알았어요 = I realized / I found out

So the structure is basically:

[After hearing the announcement, only then] [I realized] [that the platform had changed].

-고 나서야 means more than just after doing something.

• -고 나서 = after doing
• -고 나서야 = only after doing, not until doing that

So:

• 안내방송을 듣고 나서 알았어요 = I found out after hearing the announcement
• 안내방송을 듣고 나서야 알았어요 = I only found out after hearing the announcement

The 야 adds a feeling of lateness or emphasis, like only then.

Both patterns can express something like only after hearing, but they are not exactly the same in feel.

• 듣고 나서야 emphasizes the sequence clearly: first I heard it, and only after that did I realize it.
• 들어서야 can also work, but it often feels a bit more compressed and can sound slightly different in nuance depending on context.

For learners, it is safest to understand this sentence as using a very common pattern:

• V-고 나서야 = only after V-ing

So 안내방송을 듣고 나서야 is very natural.

안내방송 means a public announcement, especially one made over a speaker system in places like:

• train stations
• subways
• airports
• buses

It is made of:

• 안내 = guidance / information
• 방송 = broadcast / announcement

So in this sentence, it means the station announcement the speaker heard.

Because 승강장 is the subject of the embedded clause 승강장이 바뀌다.

That smaller clause means:

• 승강장이 바뀌다 = the platform changes / has changed

Here, the platform is the thing that changed, so it takes the subject marker 이/가.

Then that whole clause becomes the object of 알았어요:

• 승강장이 바뀐 걸 알았어요
• I realized that the platform had changed

So:

• 승강장이 = subject inside the quoted/embedded idea
• 걸 = the thing realized

바뀐 is the noun-modifying form of 바뀌다.

• dictionary form: 바뀌다 = to be changed
• modifier form: 바뀐 = changed / that changed / that has changed

It comes from the past/adnominal form, so it describes a noun-like idea:

• 승강장이 바뀐 것 = the fact that the platform changed / that the platform had changed

So yes, it carries a past/completed sense.

This is a very common question.

• 바꾸다 = to change something
• 바뀌다 = to be changed

Examples:

• 승강장을 바꿨어요 = Someone changed the platform
• 승강장이 바뀌었어요 = The platform was changed / The platform changed

In your sentence, 바뀌다 is used because the speaker is talking about the platform being different, not about who changed it.

걸 is a spoken contraction of 것을.

• 것 turns a clause into a noun-like thing: the fact that...
• 을 marks it as the object

So:

• 승강장이 바뀐 것을 알았어요 = I realized that the platform had changed
• 승강장이 바뀐 걸 알았어요 = same meaning, but more natural in everyday speech

This contraction is very common:

• 것을 → 걸
• 것이 → 게
• 것은 → 건

Because the realization happened at a specific point in the past.

알다 can mean:

• to know
• to realize / find out, depending on context

Here, 알았어요 does not just mean I knew. It means:

• I realized
• I found out

So the sentence is describing the moment the speaker came to know this information.

Korean often does not need a separate word exactly like English that.

Instead, it often uses a clause plus 것:

• 승강장이 바뀐 것 = the fact that the platform changed

So the that idea is built into the clause structure itself.

English:

• I realized that the platform had changed.

Korean:

• I realized the fact of the platform having changed.

That sounds unnatural in English, but it helps show how Korean is structuring it.

Yes, you could, and it would also be natural.

Compare:

• 승강장이 바뀐 걸 알았어요
• 승강장이 바뀌었다는 걸 알았어요

Both mean roughly I realized that the platform had changed.

The second one uses -다는 것, which is a very common way to mark reported or stated content. It can sound a bit more explicit as the fact that it changed.

The version in your sentence, 바뀐 걸, is very natural and conversational.

Because there are two different verbs involved:

1. 듣다 takes 안내방송을

   • 안내방송을 듣고 = after hearing the announcement

2. 알다 takes 승강장이 바뀐 걸

   • 승강장이 바뀐 걸 알았어요 = realized that the platform had changed

So the sentence contains two clauses:

• 안내방송을 듣고 나서야
• 승강장이 바뀐 걸 알았어요

Each verb has its own object.

Yes, to some extent.

The original sentence is very natural:

• 안내방송을 듣고 나서야 승강장이 바뀐 걸 알았어요.

You could also say:

• 승강장이 바뀐 걸 안내방송을 듣고 나서야 알았어요.

This puts more focus on what was realized.

Korean word order is flexible, but the original version is smooth and easy to process because it presents the time/condition first and then the realization.

Yes, very much so.

The key clue is -고 나서야.

It suggests:

• the speaker did not know before
• they only realized it after hearing the announcement
• there is a nuance of finally, belatedly, or not until then

So the sentence does not just report order of events. It also hints that the speaker had been unaware until that moment.

알았어요 is in the 해요체 polite style.

Levels:

• 알았다 = plain style
• 알았어요 = polite everyday style
• 알았습니다 = formal polite style

So this sentence sounds polite and natural for everyday conversation, but not especially formal.

Yes. It sounds natural and realistic, especially in a situation like a subway or train station.

It is a good example of everyday Korean because it includes:

• a common public-transport word: 안내방송
• a very useful pattern: -고 나서야
• a common spoken contraction: 걸
• a natural realization verb: 알았어요

So it is not textbook-stiff; it sounds like something a real person could say.