hyudaeponi uija araee tteoreojin jul moreugo gyesok chajasseoyo.

Because 휴대폰 is the subject of 떨어지다 in the first part of the sentence.

• 떨어지다 = to fall
• Something falls on its own, so it takes a subject marker, not an object marker

So 휴대폰이 떨어진 means the phone fell or the phone having fallen.

If you used 휴대폰을, it would sound like the phone is the object of some action, but 떨어지다 does not work that way.

의자 아래에 means under the chair.

Breakdown:

• 의자 = chair
• 아래 = below, under
• 에 = location marker

Here, 에 marks the place where the phone ended up after falling. With verbs like 떨어지다, 에 is commonly used to show the destination or location.

So:

• 의자 아래에 떨어지다 = to fall under the chair

떨어진 줄 means the fact that it had fallen or that it had fallen.

This comes from:

• 떨어지다 = to fall
• 떨어진 = fallen, having fallen
• 줄 = a bound noun used in expressions about what someone thinks, knows, or assumes

So 떨어진 줄 모르고 literally means something like:

• not knowing that it had fallen

This 줄 grammar is very common in Korean when talking about mistaken beliefs, assumptions, or lack of awareness.

-ㄴ/은 줄 모르고 means not knowing that... or without realizing that...

In this sentence:

• 떨어진 줄 모르고
• = not realizing that it had fallen

Structure:

• verb in modifier form + 줄 모르고

Examples:

• 온 줄 몰랐어요 = I did not know that someone had come
• 끝난 줄 알았어요 = I thought it was over
• 휴대폰이 떨어진 줄 모르고 = not realizing the phone had fallen

This pattern is often used when someone acts based on not knowing something.

Both can be possible in some situations, but 모르고 is very natural here because it connects two actions in sequence:

1. not realizing the phone had fallen
2. continuing to look for it

So 모르고 계속 찾았어요 means:

• not realizing it, I kept looking for it

Using 몰라서 would focus more directly on cause:

• because I did not know, I kept looking

That is not wrong in many contexts, but 모르고 often sounds smoother and more natural for doing something without knowing.

A similar English feeling is:

• I kept looking for it, not realizing it had fallen under the chair

Here, 찾았어요 means looked for.

This is a very common point of confusion because 찾다 can mean:

• to look for
• to find

In this sentence, because of 계속 and the context, it clearly means kept looking for.

Why?

• If the speaker had already found it, that would not fit well with 계속 찾았어요
• The sentence describes someone searching because they did not realize where the phone was

So here:

• 계속 찾았어요 = kept searching for it

Yes, the object is omitted, and it is understood from context.

A fuller version could be:

• 휴대폰이 의자 아래에 떨어진 줄 모르고 휴대폰을 계속 찾았어요.

But Korean often leaves out words that are already obvious. Since 휴대폰 was just mentioned, repeating it is unnecessary.

So the natural sentence simply leaves it out:

• ...모르고 계속 찾았어요
• = ...and kept looking for it

This kind of omission is extremely common in Korean.

Because the first part is not a complete sentence by itself. It is a clause modifying 줄.

Compare:

• 휴대폰이 떨어졌어요 = The phone fell
• 휴대폰이 떨어진 줄 = the fact that the phone fell

In Korean, when a verb modifies a noun, it changes into a modifier form:

• 떨어지다 → 떨어진

So 떨어진 줄 모르고 is not a separate past-tense sentence ending. It is a grammatical unit meaning:

• not knowing that it had fallen

The main sentence ends with 찾았어요, and that is where the overall sentence gets its final tense and politeness.

The subject is omitted because it is understood from context.

In English, we often need to say:

• I kept looking for it
• he kept looking for it
• she kept looking for it

In Korean, the subject is often left out if it is obvious. This sentence could mean:

• I kept looking for it
• he/she kept looking for it
• someone kept looking for it

depending on context.

So the sentence does not explicitly tell you who the searcher is. You figure that out from the conversation or situation.

Yes. 의자 밑에 is also very natural and often sounds even more everyday in this kind of situation.

• 아래 = below, under
• 밑 = under, underneath, bottom

For a physical object like a chair, both are possible:

• 의자 아래에
• 의자 밑에

In many daily situations, 밑 can feel a bit more concrete and physical, while 아래 can feel slightly broader or more neutral. But both are correct here.

Yes, but the nuance changes slightly.

• 떨어진 줄 모르고 focuses on the event: not realizing it had fallen
• 떨어져 있는 줄 모르고 focuses more on the state/result: not realizing it was lying there after falling

So:

• 떨어진 줄 모르고 = did not realize it had fallen
• 떨어져 있는 줄 모르고 = did not realize it was there on the floor / in a fallen state

In your sentence, 떨어진 줄 모르고 is very natural because the important idea is that the speaker did not realize what happened to the phone.