badagi mikkeureoun jul moreugo ttwidaga neomeojil ppeonhaesseo.

Here, 줄 means something like the fact that..., that..., or thinking that....

So 미끄러운 줄 모르고 literally means:

• not knowing that it was slippery
• without realizing that the floor was slippery

This pattern is very common:

• V/A-(으)ㄴ/는 줄 알다 = to think/know that ...
• V/A-(으)ㄴ/는 줄 모르다 = to not know/realize that ...

Examples:

• 오는 줄 알았어. = I thought he was coming.
• 비싼 줄 몰랐어. = I didn’t know it was expensive.

In your sentence, 미끄러운 줄 모르고 connects that lack of awareness to what happened next.

Because 미끄럽다 is a descriptive verb (often called an adjective in Korean grammar). Descriptive verbs use the -ㄴ/은 modifier before a noun like 줄.

So:

• 미끄럽다 → stem 미끄럽-
• with noun-modifying form → 미끄러운
• therefore: 미끄러운 줄

Compare:

• 예쁘다 → 예쁜 줄
• 작다 → 작은 줄
• 좋다 → 좋은 줄

By contrast, action verbs usually take -는:

• 오다 → 오는 줄
• 먹다 → 먹는 줄

So 미끄러운 줄 is the correct descriptive-verb form.

-고 connects clauses, often like and, and then, or while. In this sentence, 모르고 means:

• not knowing and...
• without realizing and then...
• more naturally: without realizing

So:

• 바닥이 미끄러운 줄 모르고 = not realizing that the floor was slippery
• then the next action follows: 뛰다가 넘어질 뻔했어

This -고 often links the background circumstance to what happened afterward.

Both can work in similar situations, but they feel a little different.

• 모르고 = without knowing / not realizing, and then...
• 몰라서 = because I didn’t know...

In this sentence, 모르고 emphasizes the state of being unaware while the next action happened. It sounds very natural when describing a mistaken action followed by a result.

So:

• 바닥이 미끄러운 줄 모르고 뛰다가...
  = I ran without realizing the floor was slippery...

If you said 몰라서, it would sound more explicitly causal:

• 바닥이 미끄러운 줄 몰라서...
  = Because I didn’t know the floor was slippery...

Both can be understandable, but 모르고 fits very naturally with accidental or mistaken behavior.

-다가 means while doing, in the middle of doing, or sometimes then / and while doing that.

So 뛰다가 means:

• while running
• in the middle of running

It shows that the next event happened during that action.

So the structure is:

• 뛰다 = to run
• 뛰다가 = while running / while I was running

In the sentence:

• 뛰다가 넘어질 뻔했어
  = I almost fell while running

This is a very common use of -다가 to show an interruption or an event that occurs during another action.

Examples:

• 걷다가 친구를 만났어. = I met a friend while walking.
• 공부하다가 잠들었어. = I fell asleep while studying.

Good question. Both can relate two actions, but they are not the same.

• -면서 = doing two actions at the same time
• -다가 = something happens during an action, often as an interruption or change

In your sentence, 뛰다가 넘어질 뻔했어 means the almost-falling happened in the middle of running. That fits -다가 very well.

If you used -면서, it would suggest more direct simultaneity:

• 뛰면서... = while running...

But 넘어질 뻔했어 is more like an event that occurred during the running, not simply another simultaneous action. So -다가 sounds more natural.

-ㄹ/을 뻔하다 means almost did or came close to doing something.

So:

• 넘어지다 = to fall
• 넘어질 뻔하다 = to almost fall

In the sentence:

• 넘어질 뻔했어 = I almost fell

Important nuance: This grammar usually implies that the event did not actually happen.

So 넘어질 뻔했어 means:

• I nearly fell
• I was close to falling
• but I didn’t actually fall

Examples:

• 지각할 뻔했어. = I almost was late.
• 울 뻔했어. = I almost cried.
• 죽을 뻔했어. = I almost died.

Because -ㄹ/을 뻔하다 attaches to the verb stem and expresses something that was about to happen but didn’t.

So the pattern is:

• verb stem + -ㄹ/을 뻔하다

For 넘어지다:

• stem: 넘어지-
• add -ㄹ 뻔하다
• 넘어질 뻔하다

You do not use the past-tense form of the main verb before 뻔하다.

Compare:

• 갈 뻔했어 = I almost went
• 먹을 뻔했어 = I almost ate it
• 놓칠 뻔했어 = I almost missed it

The past tense is carried by 뻔했어, not by the main verb itself.

이/가 marks the subject. Here, 바닥이 미끄러운 줄 means that the floor was slippery.

So 바닥이 is simply identifying the floor as the thing that is slippery.

If you used 바닥은, it would add a contrastive or topical nuance, something like:

• as for the floor, it was slippery
• possibly contrasting it with something else

In this sentence, 바닥이 is the neutral and natural choice.

So:

• 바닥이 미끄럽다 = the floor is slippery
• 바닥은 미끄럽다 = as for the floor, it is slippery / the floor, at least, is slippery

Yes, grammatically it is.

Korean often uses a verb or descriptive verb in a noun-modifying form before a noun. Here:

• 미끄러운 = slippery
• 줄 = fact / idea / expectation in this pattern

So 미끄러운 줄 is literally something like:

• the fact that it was slippery
• that it was slippery

Even though that sounds a little unnatural in English, it is a very normal Korean structure.

It breaks down like this:

• 바닥이 = the floor
• 미끄러운 줄 = that it was slippery
• 모르고 = not realizing / without knowing
• 뛰다가 = while running
• 넘어질 뻔했어 = almost fell

So the structure is basically:

Without realizing that the floor was slippery, I was running and almost fell.

Or more naturally:

I almost fell while running because I didn’t realize the floor was slippery.

Yes, in normal conversation that is the natural interpretation.

Korean often omits the subject when it is understood from context. Since the sentence ends with 넘어질 뻔했어, the implied subject is usually I in casual speech:

• I didn’t realize the floor was slippery
• I was running
• I almost fell

If the speaker wanted to make another subject explicit, they would normally mention it.

It is informal casual speech.

The basic dictionary form is:

• 넘어질 뻔하다

In casual past tense:

• 넘어질 뻔했어

This style is used with:

• friends
• younger people
• close acquaintances
• casual conversation

A more polite version would be:

• 바닥이 미끄러운 줄 모르고 뛰다가 넘어질 뻔했어요.

So the sentence as written sounds natural and conversational.