geokjeongi manhaseo jami an wasseoyo.

Because 많다 works like to be many / to be a lot, so the thing that is “many” takes a subject marker: 이/가.

• 걱정이 많다 = literally worries are many
• Natural English: to have a lot of worries / to be very worried

So:

• 걱정이 많아서 = because there were many worries / because I had a lot on my mind

Using 걱정을 많아서 would be ungrammatical here.

Here -아서 / -어서 connects two clauses and gives a reason or cause.

So:

• 걱정이 많아서 = because I was worried a lot / because I had many worries
• 잠이 안 왔어요 = sleep didn’t come

Together:

• 걱정이 많아서 잠이 안 왔어요 = Because I was so worried, I couldn’t sleep

In this sentence, -아서 is basically functioning like because or so.

Because Korean often expresses sleep as something that comes or doesn’t come.

• 잠이 오다 = sleep comes = to feel sleepy / to be able to fall asleep
• 잠이 안 오다 = sleep doesn’t come = to not be able to sleep / not feel sleepy

So 잠 is treated as the subject of 오다, which is why it uses 이/가:

• 잠이 안 왔어요 = literally sleep didn’t come

This is a very natural Korean expression.

That is just the natural Korean way to describe this situation.

There is a nuance difference:

• 잠이 안 왔어요 = sleep wouldn’t come / I couldn’t fall asleep
• 잠을 못 잤어요 = I couldn’t sleep / I didn’t get sleep

The sentence you gave focuses on the feeling of trying to sleep, but sleep not coming. It sounds very natural when worry, stress, or nervousness is the reason.

Because the speaker is describing what happened in the past.

• 잠이 안 와요 = I can’t sleep / sleep isn’t coming
  usually about right now
• 잠이 안 왔어요 = I couldn’t sleep / sleep didn’t come
  about a past situation

So this sentence means the worrying happened, and as a result, the speaker was unable to sleep.

In Korean, when one clause explains the reason for a past event, the first clause often stays in its basic connective form:

• 걱정이 많아서 잠이 안 왔어요

This is natural and common.

Even though the whole situation is in the past, Korean often lets the final verb carry the tense clearly enough. So the meaning is understood as:

• I had a lot of worries, so I couldn’t sleep

A form like 많았어서 is much less natural here.

Yes. Korean very often omits subjects when they are obvious from context.

So even though the sentence does not say 저는 or 제가, the meaning is understood as something like:

• I had a lot of worries, so I couldn’t sleep

This omission is extremely common in Korean and sounds natural.

Both can relate to worrying, but they are structured differently.

• 걱정이 많다 = to have many worries / to be full of worries
• 걱정을 많이 하다 = to worry a lot

So:

• 걱정이 많아서 잠이 안 왔어요 = I had a lot on my mind, so I couldn’t sleep
• 걱정을 많이 해서 잠이 안 왔어요 = I worried a lot, so I couldn’t sleep

Both are possible, but 걱정이 많아서 feels a little more like describing the speaker’s mental state, while 걱정을 많이 해서 emphasizes the action of worrying.

Yes. 걱정 can cover ideas like:

• worry
• concern
• anxiety

So depending on context, the sentence could sound like:

• I was so worried that I couldn’t sleep
• I had so much on my mind that I couldn’t sleep
• I was anxious and couldn’t sleep

The exact English wording depends on tone and context, even though the Korean sentence itself is straightforward.

It is the polite casual style, often called 해요체.

• 왔어요 = polite and natural in everyday conversation
• Not formal-stiff, but respectful enough for most situations

Compare:

• 잠이 안 왔어요 = polite everyday speech
• 잠이 안 왔어 = casual speech to close friends
• 잠이 안 왔습니다 = formal style

So the full sentence is polite, natural everyday Korean.

Yes, and it would sound natural, but the nuance changes a little.

• 걱정이 많아서 잠이 안 왔어요 = I had a lot of worries, so I couldn’t sleep
• 걱정돼서 잠이 안 왔어요 = I was worried, so I couldn’t sleep

걱정이 많아서 emphasizes having many worries / a lot on your mind.
걱정돼서 emphasizes the feeling I was worried.

Both are common, but your original sentence highlights the idea of many worries piling up.

Often yes, but context matters.

잠이 안 오다 can mean:

• sleep won’t come
• I’m not getting sleepy
• I can’t fall asleep

In a sentence like this, because there is a reason beforehand:

• 걱정이 많아서 잠이 안 왔어요

the meaning is clearly:

• I couldn’t fall asleep because I was worried

So here it is not just a neutral I wasn’t sleepy. It strongly suggests that the speaker wanted to sleep, but couldn’t.

Yes. A very literal translation is:

• Because there were many worries, sleep didn’t come

But more natural English versions include:

• I had a lot on my mind, so I couldn’t sleep.
• I was so worried that I couldn’t sleep.
• Because I was worrying so much, I couldn’t fall asleep.

These all match the Korean well, depending on the tone you want.