maneul naemsaega ganghaeseo changmuneul yeoreo nwasseo.

Korean often puts two nouns together directly, so 마늘 냄새 means garlic smell or the smell of garlic.

• 마늘 = garlic
• 냄새 = smell

You can say 마늘의 냄새, but it sounds more explicit or literary. In everyday Korean, 마늘 냄새 is much more natural.

Because 강하다 works like to be strong, so the thing that is strong is marked as the subject.

Here, the sentence is not saying something does the smell. It is saying:

• 냄새가 강하다 = the smell is strong

So 냄새 takes the subject marker 가.

This is very common with descriptive verbs/adjectives in Korean:

• 날씨가 좋다 = the weather is good
• 소리가 크다 = the sound is loud
• 냄새가 강하다 = the smell is strong

강해서 comes from 강하다 + -아서/어서, which connects clauses and often means because, so, or since.

With 하다 verbs/descriptive verbs, -여서 often contracts:

• 강하다 → 강하여서 → 강해서

So 냄새가 강해서 means because the smell is strong.

In this sentence, it gives the reason for the second action:

• Because the garlic smell is strong, I left the window open.

열어 놨어 is 열다 + -어 놓다.

The pattern -어 놓다 means:

• do something
• and leave it in that state

So 창문을 열어 놨어 does not just mean opened the window. It means:

• opened the window and left it open

This pattern often carries the idea of a resulting state that still matters now.

The difference is nuance:

• 열었어 = opened it
• 열어 놨어 = opened it and left it open

So 열어 놨어 focuses on the current result.

For example:

• 창문을 열었어. = I opened the window.
  
  • This only tells you the action happened.
• 창문을 열어 놨어. = I opened the window and left it open.
  
  • This tells you the window is probably still open now.

That is why -어 놓다 is very common when the ongoing state matters.

Yes. 놨어 is a contraction of 놓았어.

So:

• 열어 놓았어 → 열어 놨어

This contraction is very common in everyday speech and casual writing. It is completely normal.

Because 창문 is the direct object of 열다.

• 창문을 열다 = to open the window

So 을/를 marks the thing being opened.

Compare:

• 문을 열다 = open the door
• 창문을 닫다 = close the window

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

So 창문을 열어 놨어 usually implies something like:

• I left the window open
• we left the window open

The exact subject depends on the situation. In everyday Korean, this kind of omission is very normal.

It is casual/informal speech.

The sentence ends in -어, which is used with:

• friends
• people younger than you
• very casual situations

A polite version would be:

• 마늘 냄새가 강해서 창문을 열어 놨어요.

A less contracted version would be:

• 마늘 냄새가 강해서 창문을 열어 놓았어요.

Not necessarily. In many sentences, -아서/어서 simply gives a natural reason or background.

Here, 강해서 sounds like:

• because the smell was strong
• since the smell was strong

It feels natural and explanatory, not overly dramatic. It is one of the most common ways in Korean to connect a reason to a result.