gapjagi keun soriga naseo jeongmal nollasseoyo.

갑자기 means suddenly or all of a sudden.

In this sentence, it comes at the beginning because it sets the scene for the whole event:

• 갑자기 큰 소리가 나서 정말 놀랐어요.
• Suddenly, there was a loud noise, so I was really surprised.

In Korean, adverbs like 갑자기 are often placed near the beginning of the clause, but they can sometimes move around as long as the sentence still sounds natural.

For example:

• 큰 소리가 갑자기 나서 정말 놀랐어요.

That is also understandable, though the original sentence sounds very natural.

큰 is the noun-modifying form of 크다 (to be big).

When a descriptive verb/adjective in Korean directly modifies a noun, it changes form:

• 크다 = to be big
• 큰 = big, when placed before a noun

So:

• 큰 소리 = a big sound → more naturally, a loud noise

Why not the others?

• 크다 소리 is not grammatical.
• 크는 소리 is usually not correct here, because 크다 is a descriptive verb, and this kind of adjective usually uses the -(으)ㄴ form before nouns.

More examples:

• 작은 방 = a small room
• 예쁜 꽃 = a pretty flower
• 좋은 생각 = a good idea

Yes, literally it is big sound, but in natural English that usually becomes loud sound or loud noise.

In Korean, 크다 is often used where English would say loud for sounds.

So:

• 큰 소리 = loud sound / loud noise
• 큰 음악 소리 = loud music
• 소리가 크다 = the sound is loud

This is very normal Korean usage.

Because 소리 is the thing that is happening/occurring, not the direct object of an action.

Here the verb is 나다, which often means to come out, to occur, or to be produced depending on context.

So in:

• 큰 소리가 나다

the sound is the subject of 나다, which is why it takes 이/가:

• 소리가 나다 = a sound occurs / a sound comes out / a sound is heard

If you used 소리를, that would suggest the sound is the object of another verb, such as:

• 큰 소리를 들었어요. = I heard a loud sound.

So the difference is:

• 큰 소리가 났어요. = A loud sound occurred.
• 큰 소리를 들었어요. = I heard a loud sound.

This is a very common Korean pattern.

나다 is used for things that appear, occur, or come out, including:

• 소리가 나다 = a sound occurs / a sound comes out
• 냄새가 나다 = a smell comes out / there is a smell
• 문제가 나다 = a problem occurs
• 불이 나다 = a fire breaks out

So 큰 소리가 나서 literally feels like:

• because a loud sound came out/occurred

Even though that sounds unusual in English, it is perfectly natural in Korean.

나서 is 나다 + -아서/어서, which connects two clauses.

Here it means something like:

• because a loud sound happened
• and then, because of that, I was surprised

So:

• 큰 소리가 나서 정말 놀랐어요.

can be understood as:

• A loud noise happened, so I was really surprised.

This -아서/어서 ending often shows:

1. cause/reason
   
   • 비가 와서 못 갔어요. = It rained, so I couldn't go.
2. sequence
   
   • 집에 가서 잤어요. = I went home and slept.

In this sentence, the meaning is mainly cause.

Yes. That is a very natural translation.

Korean often puts the cause first and the result second:

• 갑자기 큰 소리가 나서 = because a loud noise suddenly occurred
• 정말 놀랐어요 = I was really surprised

So the overall logic is:

cause → result

That is one reason the sentence may feel a little backward compared with some English sentences, but it is very normal in Korean.

The base form is 놀라다, which means to be surprised or to get startled.

The form 놀랐어요 breaks down like this:

• 놀라다 → stem 놀라-
• past tense: 놀랐-
• polite ending: -어요

So:

• 놀랐어요 = was surprised / got surprised / was startled

This is the standard polite spoken style.

Related forms:

• 놀라요 = am/is surprised
• 놀랐어요 = was surprised
• 놀랍니다 = am/is surprised (formal)
• 놀랐습니다 = was surprised (formal)

Yes, the subject is omitted, which is very common in Korean.

The sentence does not explicitly say I, but 놀랐어요 strongly implies the speaker is talking about their own experience:

• (저는) 갑자기 큰 소리가 나서 정말 놀랐어요.

Korean often leaves out subjects when they are obvious from context.

So even without 저는, the listener naturally understands:

• I was really surprised.

정말 means really, truly, or very.

Here it intensifies 놀랐어요:

• 정말 놀랐어요 = I was really surprised

It makes the reaction stronger.

Similar words include:

• 아주 = very
• 진짜 = really / seriously / for real
• 너무 = very / too

For example:

• 정말 놀랐어요. = I was really surprised.
• 진짜 놀랐어요. = I was really surprised. / I was seriously startled.

정말 sounds natural and neutral here.

Yes, you can, but the nuance is a little different.

• 큰 소리가 나서 = because a loud noise occurred
• 큰 소리를 들어서 = because I heard a loud noise

The original sentence focuses more on the event itself: a loud noise suddenly happened.

The version with 들어서 focuses more on your perception of it: you heard the loud noise.

Both can work, but the original is very natural because Korean often describes these situations with 소리가 나다.

A natural pronunciation is close to:

• 크ㄴ 소리가 나서

A few helpful notes:

1. 큰 is from 크다, but when spoken it sounds like 큰 with a light n at the end.
2. 소리가 is pronounced smoothly as 소리가.
3. The whole phrase flows together naturally:
   • 큰 소리가 나서

You do not need to force a pause between every word. In natural speech, Korean connects smoothly.

A learner-friendly approximation might be:

• keun soriga naseo

But it is best to listen to native audio if possible, since Korean rhythm matters a lot.

The ending -어요 makes the sentence polite informal, which is very common in everyday conversation.

Original:

• 갑자기 큰 소리가 나서 정말 놀랐어요.

Other styles:

1. Plain/casual

   • 갑자기 큰 소리가 나서 정말 놀랐어.
   • Used with close friends, younger people, or in casual speech.

2. Formal polite

   • 갑자기 큰 소리가 나서 정말 놀랐습니다.
   • Used in more formal situations.

3. Dictionary form

   • 갑자기 큰 소리가 나서 정말 놀랐다.
   • Often seen in writing, narration, or explanations.

So the original sentence is in a very useful everyday style.