mesijireul dasi ilggo naseoya naega silsuhan geol arasseoyo.

-고 나서야 adds a strong sense of “only after” or “not until after.”

So:

• 읽고 나서 = after reading
• 읽고 나서야 = only after reading / not until after reading

In this sentence, it emphasizes that the speaker did not realize the mistake before, and the realization happened only after rereading the message.

So the nuance is stronger than a simple sequence of events.

Good question. The difference is mostly emphasis.

• 메시지를 다시 읽고 나서 내가 실수한 걸 알았어요.
  = After reading the message again, I realized I had made a mistake.

• 메시지를 다시 읽고 나서야 내가 실수한 걸 알았어요.
  = It was only after reading the message again that I realized I had made a mistake.

The second version suggests:

• the speaker hadn’t realized it earlier
• rereading the message was the key moment that led to the realization

So -야 is what gives that stronger “only then” feeling.

다시 means again.

So 메시지를 다시 읽고 means:

• 메시지를 읽고 = reading the message
• 메시지를 다시 읽고 = reading the message again

This tells us the speaker had already read the message once, but after reading it again, they noticed the mistake.

Here, 내가 is marking the subject inside the embedded clause: I made a mistake.

The part 내가 실수한 걸 means something like:

• the fact that I made a mistake
• that I had made a mistake

Using 가 is natural because it identifies who made the mistake.

If you said 나는, it would sound more like setting up a topic: as for me... That is possible in some contexts, but here 내가 is more natural because it is part of the clause being realized.

So:

• 내가 실수한 걸 알았어요 = I realized that I had made a mistake.

Because it modifies an implied noun, usually 것.

• 실수하다 = to make a mistake
• 실수한 것 = the thing/fact that someone made a mistake

So 실수한 걸 comes from:

• 실수한 것을

This is a very common way in Korean to turn a clause into “the fact that...”

So:

• 내가 실수한 것 = the fact that I made a mistake
• 내가 실수한 걸 알았어요 = I realized that I had made a mistake

The past modifier -한 shows that the mistake had already happened before the realization.

걸 is a contraction of 것을.

So:

• 내가 실수한 것을 알았어요
• 내가 실수한 걸 알았어요

mean the same thing.

Breakdown:

• 것 = thing / fact
• 을 = object particle
• 것을 → 걸 in casual speech and very common everyday writing

Here it works like “that” or “the fact that” in English.

So 내가 실수한 걸 알았어요 literally means something like: I knew/realized the fact that I had made a mistake.

Because in Korean, 알았어요 often means “came to know” or “realized.”

Compare:

• 알아요 = I know
• 알았어요 = I found out / I realized / I came to know

In this sentence, the speaker is talking about the moment of realization, not just the state of knowing.

So:

• 내가 실수한 걸 알아요 = I know I made a mistake
• 내가 실수한 걸 알았어요 = I realized I had made a mistake

This use of the past tense is very common with verbs like 알다 when talking about discovering something.

Because 메시지 is the object of 읽다.

• 읽다 = to read
• What did the speaker read? 메시지
• So it takes the object particle -를

That gives us:

• 메시지를 읽다 = to read the message

This is straightforward object marking.

Yes, very often it can be omitted if the context is clear.

For example:

• 메시지를 다시 읽고 나서야 실수한 걸 알았어요.

This would still be understandable as: Only after rereading the message did I realize I had made a mistake.

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

However, keeping 내가 can help:

• make the sentence clearer
• emphasize that I was the one who made the mistake

So both are possible, but the original sentence is perfectly natural.

Yes. 내가 and 제가 both mean I, but they differ in politeness/register.

• 내가 = plain, non-humble
• 제가 = humble/polite

Since the sentence ends with 알았어요, which is polite speech, some learners wonder whether 제가 should be used. In real Korean, both can appear depending on context.

• 내가 sounds natural in many everyday situations
• 제가 would sound a bit more polite or formal

So you could also say:

• 메시지를 다시 읽고 나서야 제가 실수한 걸 알았어요.

That would also be correct.

It helps to break it into parts:

1. 내가 = I
2. 실수한 = having made a mistake
3. 것을 / 걸 = the fact / thing
4. 알았어요 = realized / came to know

So the pattern is:

[clause in modifier form] + 것(을) 알다

This often means:

• to know that...
• to realize that...

Examples:

• 그가 떠난 걸 알았어요. = I found out that he left.
• 문제가 있다는 걸 알았어요. = I realized that there was a problem.

So your sentence follows a very common grammar pattern.

Literally, maybe yes, but that would usually sound unnatural in English.

In Korean, 알았어요 often covers both:

• knew
• found out
• realized

In this context, the best English translation is usually:

• I realized I had made a mistake
• Only after rereading the message did I realize I had made a mistake

That captures the actual meaning much better than I knew that I made a mistake.

The current order is very natural:

• 메시지를 다시 읽고 나서야 = only after rereading the message
• 내가 실수한 걸 알았어요 = I realized I had made a mistake

Korean usually puts background information first and the main action later. So this sentence flows naturally from:

1. the condition/time
2. the realization

Because Korean word order is flexible, some parts can move around, but this version is standard and easy to understand.

걸 is very common in speech and also common in informal or semi-formal writing.

Compare:

• 것을 = more full, careful, slightly more formal
• 걸 = contracted, natural, conversational

So in everyday Korean, 걸 알았어요 sounds very normal.

If you were writing something very formal, you might choose 것을, but in ordinary use 걸 is completely natural.