nagyeop ttaemune baesuguga makhyeossseupnida.

낙엽 means fallen leaves.

• 낙 = falling
• 엽 = leaf

In everyday Korean, you can just learn it as one word meaning fallen leaves, especially the leaves you see on the ground in autumn.

배수구 means drain or drain opening.

It is the place where water flows out, such as:

• a street drain
• a gutter drain
• a floor drain

So in this sentence, it is not a general pipe system, but the actual drain that got clogged.

때문에 means because of.

In this sentence:

• 낙엽 때문에 = because of the fallen leaves

It is commonly used after a noun to show a reason or cause.

Structure:

• noun + 때문에 = because of noun

So:

• 비 때문에 = because of the rain
• 교통 때문에 = because of traffic
• 낙엽 때문에 = because of fallen leaves

It often sounds natural when talking about causes, especially ones that lead to a problem or result.

Because 배수구 is the thing that became clogged, so it is the subject of the verb.

• 배수구가 막혔습니다 = the drain got clogged

Here, 막히다 means to be clogged / to get blocked, so the drain is not the object being acted on by someone in the sentence. It is the thing experiencing the state.

If you used 를, it would suggest a different structure, usually with an active verb like 막다:

• 낙엽이 배수구를 막았습니다 = The fallen leaves blocked the drain

That is also a correct sentence, but it is grammatically different.

막혔습니다 comes from 막히다, which means to be blocked, to get blocked, or to be clogged.

So:

• 막히다 = be blocked / get clogged
• 막혔습니다 = was blocked / got clogged

It is related to 막다, which means to block.

Compare:

• 누군가가 길을 막았습니다.
  = Someone blocked the road.
• 길이 막혔습니다.
  = The road is blocked / got blocked.

For English speakers, it can feel similar to a passive, but in Korean it is often just the normal verb used when something ends up blocked.

Korean usually puts the reason first and the verb at the end.

So the sentence is:

• 낙엽 때문에 = because of fallen leaves
• 배수구가 = the drain
• 막혔습니다 = got clogged

A very literal order would be:

Because of fallen leaves, the drain got clogged.

That is a very normal Korean sentence pattern:

• cause/reason
• subject
• verb

막혔습니다 is in the formal polite style, often called the -습니다 style.

It is used in:

• formal conversation
• announcements
• reports
• writing
• speaking politely to someone you do not know well

Other versions:

• 막혔어요 = polite, less formal
• 막혔어 = casual

So the same sentence can appear as:

• 낙엽 때문에 배수구가 막혔습니다.
  formal polite
• 낙엽 때문에 배수구가 막혔어요.
  standard polite
• 낙엽 때문에 배수구가 막혔어.
  casual

Yes. That sentence is also correct.

Compare the two:

• 낙엽 때문에 배수구가 막혔습니다.
  = The drain got clogged because of fallen leaves.
• 낙엽이 배수구를 막았습니다.
  = The fallen leaves blocked the drain.

The difference is mainly in perspective:

1. 낙엽 때문에 배수구가 막혔습니다

   • focuses more on the result
   • sounds natural when explaining what happened

2. 낙엽이 배수구를 막았습니다

   • focuses more on the leaves as the thing doing the blocking

Both are fine, but the original sentence sounds a little more natural in many everyday situations.

Not only, but it is very often used for neutral or negative causes.

For example:

• 비 때문에 늦었어요.
  = I was late because of the rain.
• 감기 때문에 학교에 못 갔어요.
  = I couldn't go to school because of a cold.

You can use it in positive situations too, but depending on the context, other expressions like 덕분에 may sound more natural for a positive cause.

Compare:

• 친구 때문에 많이 웃었어요.
  = I laughed a lot because of my friend.
  Possible, but can depend on context.
• 친구 덕분에 많이 웃었어요.
  = Thanks to my friend, I laughed a lot.
  More clearly positive.

In your sentence, 때문에 fits very naturally because a clogged drain is an unwanted result.

A very natural everyday version would be:

• 낙엽 때문에 배수구가 막혔어요.

If you are speaking casually to a friend:

• 낙엽 때문에 배수구가 막혔어.

All three mean the same basic thing:

• 막혔습니다 = formal polite
• 막혔어요 = everyday polite
• 막혔어 = casual

낙엽 is pronounced more like 나겹 in actual speech.

That happens because of Korean sound changes.

Very roughly:

• written: 낙엽
• sounded: 나겹

You do not need to memorize every sound rule right away, but it is useful to know that Korean pronunciation often changes slightly from the spelling.