susul ttaemune oneul sane mos ga.

때문에 means because of or due to.

In this sentence, 수술 때문에 means because of surgery or because of the operation.

A very useful pattern is:

• noun + 때문에 = because of + noun

Examples:

• 비 때문에 못 가. = I can’t go because of the rain.
• 일 때문에 바빠. = I’m busy because of work.

With this sentence, 수술 때문에 오늘 산에 못 가. gives the reason first, then the result.

Korean often leaves out the subject when it is clear from context.

So 수술 때문에 오늘 산에 못 가. could mean:

• I can’t go to the mountain today because of surgery
• We can’t go...
• He/She can’t go...

In real conversation, the listener usually already knows who is being talked about, so Korean does not need to say the subject as often as English does.

If you wanted to include it, you could say:

• 저는 수술 때문에 오늘 산에 못 가. = As for me, I can’t go today because of surgery.

But in everyday Korean, leaving it out is very natural.

This is a very important difference.

• 못 가 = can’t go / am unable to go
• 안 가 = don’t go / won’t go

So:

• 수술 때문에 오늘 산에 못 가. = I can’t go today because of surgery.
• 오늘 산에 안 가. = I’m not going to the mountain today.

In your sentence, 못 is used because the speaker is prevented from going. It is not just a personal choice.

가 is the casual, non-polite form.

The verb is 가다 = to go.
In casual speech, 가다 becomes 가.

So:

• 못 가 = casual
• 못 가요 = polite
• 못 갑니다 = formal

Your sentence is in plain casual speech:

• 수술 때문에 오늘 산에 못 가.

Polite versions:

• 수술 때문에 오늘 산에 못 가요.
• 수술 때문에 오늘 산에 못 갑니다.

Korean often uses the present-form verb for near-future meaning when the time is already clear from context.

Because the sentence includes 오늘 (today), Korean does not need a separate future marker.

So 오늘 산에 못 가 naturally means:

• I can’t go to the mountain today
• I won’t be able to go today

This is very normal in Korean.

에 here marks a destination.

With motion verbs like 가다 (to go), 에 often means to.

So:

• 산에 가다 = to go to the mountain / to go to the mountains

This is different from English, where in, on, to, and at are all separate words. In Korean, 에 can cover several location-related meanings, and the verb helps determine the exact meaning.

With 가다, 산에 is understood as to the mountain.

It can often imply going to the mountains or going hiking, depending on context.

Literally:

• 산 = mountain
• 산에 가다 = go to the mountain

But in real Korean, 산에 가다 is also commonly used for the idea of going to the mountains for hiking or climbing.

So depending on context, this could sound like:

• I can’t go to the mountain today
• I can’t go hiking today
• I can’t go to the mountains today

Yes, Korean word order is fairly flexible, especially with time expressions like 오늘.

In your sentence:

• 수술 때문에 오늘 산에 못 가.

오늘 means today, and it naturally comes before the main verb phrase.

You may also hear:

• 오늘 수술 때문에 산에 못 가.
• 산에 오늘 못 가.

These can all be understandable, but 수술 때문에 오늘 산에 못 가 sounds very natural.

In Korean, the most important thing is that the verb comes near the end.

Korean does not use articles like a, an, and the the way English does.

So 수술 때문에 could mean:

• because of surgery
• because of the surgery
• because I have surgery
• because of an operation

The exact interpretation depends on context.

That is very common in Korean: the sentence gives the basic idea, and context fills in the details.

In the form used here, yes:

• noun + 때문에

So:

• 수술 때문에
• 비 때문에
• 일 때문에

If you want to say because with a verb clause, Korean usually uses:

• verb/adjective stem + 기 때문에

Examples:

• 아프기 때문에 못 가. = I can’t go because I’m sick.
• 늦었기 때문에 택시를 탔어. = Because I was late, I took a taxi.

So your sentence uses the noun pattern:

• 수술 때문에

Yes, it is natural.

수술 때문에 오늘 산에 못 가. sounds like normal casual spoken Korean.

That said, depending on context, a Korean speaker might also say:

• 오늘 수술이 있어서 산에 못 가.
• 수술이 있어서 오늘 산에 못 가.

These versions can sound a little more like:

• I have surgery today, so I can’t go to the mountain.

Your original sentence is still completely natural and clear.

It is in casual speech.

The ending 가 without 요 makes it informal, so it would be used with:

• close friends
• family
• younger people
• someone you speak casually with

Polite version:

• 수술 때문에 오늘 산에 못 가요.

More formal version:

• 수술 때문에 오늘 산에 못 갑니다.

So the grammar is the same, but the ending changes depending on who you are speaking to.