gapjagi joheun saenggagi deureosseoyo.

갑자기 means suddenly or all of a sudden.

It is an adverb, so it describes how the idea came to mind. Putting it at the beginning is very natural in Korean, because it sets the scene for the whole sentence:

갑자기 + 좋은 생각이 들었어요
= Suddenly, a good idea occurred to me.

You can sometimes move adverbs around in Korean, but sentence-initial 갑자기 is one of the most common and natural placements.

In Korean, words like 좋다 work like descriptive verbs, and when they directly describe a noun, they must change into a noun-modifying form.

So:

• 좋다 = to be good
• 좋은 = good, when placed before a noun

That gives:

• 좋은 생각 = a good idea / a good thought

So 좋은 is the correct form because it is modifying 생각.

This is because the expression is 생각이 들다, not 생각을 들다.

In this pattern, 생각 is treated as the thing that comes up or occurs, so it takes the subject marker 이/가.

You can think of the structure roughly like this:

• 좋은 생각이 들다
  = a good idea comes to mind = a good idea occurs

So 이 is marking 생각 as the thing that appeared in your mind.

No. Here it comes from 들다, not 듣다.

That is an important distinction:

• 듣다 = to hear / to listen
• 들다 = has several meanings, and here it means to occur to someone or to come into one’s mind

So 생각이 들다 is a common expression meaning:

• a thought occurs
• an idea comes to mind
• I get the thought that...

So 들었어요 here does not mean heard.

Korean often uses the past tense here because the moment of having the idea is seen as a completed event.

So:

• 좋은 생각이 들었어요 = a good idea occurred to me
• literally, the idea came to mind

Even if English sometimes uses the present, Korean often prefers the past when reporting that a thought suddenly came to you.

If you said:

• 좋은 생각이 들어요

that could mean a good idea is coming to me or I think I have an idea, and it sounds more immediate or in-the-moment.

Yes, in most situations it is understood as I had the idea.

Korean very often leaves out the subject when it is obvious from context. So instead of saying:

• 저는 갑자기 좋은 생각이 들었어요

people often simply say:

• 갑자기 좋은 생각이 들었어요

The listener usually understands who had the thought based on the conversation.

Depending on context, it could also refer to someone else, but without extra context, many learners first meet it with the understood subject I.

들었어요 is in the polite informal style, sometimes called 해요체.

It is very common in everyday conversation and is appropriate in many normal situations.

Related forms:

• 들었어 = casual / intimate
• 들었어요 = polite
• 들었습니다 = more formal

So this sentence is polite, natural, and conversational.

Yes. 좋은 생각이 났어요 is also very natural.

Both are common, but there is a slight nuance:

• 좋은 생각이 들었어요 = a good idea occurred to me
• 좋은 생각이 났어요 = a good idea came up / came to me

In many situations, they are very close.

A useful note:

• 생각이 나다 can also mean to remember
• 생각이 들다 often feels more like a thought occurred to me

So both work, but the exact feeling can depend on context.

You can say 좋은 생각을 했어요, but it does not mean exactly the same thing.

• 좋은 생각이 들었어요 = a good idea occurred to me
• 좋은 생각을 했어요 = I thought a good thought / I had a good thought

The first one is much more natural when you mean that an idea suddenly came to you. The second sounds more like you are describing the act of thinking.

So for an idea suddenly occurred to me, 좋은 생각이 들었어요 is the more idiomatic choice.

Yes, Korean word order is somewhat flexible, especially with adverbs. But some placements sound more natural than others.

Most natural:

• 갑자기 좋은 생각이 들었어요

Possible, but less neutral:

• 좋은 생각이 갑자기 들었어요

Starting with 갑자기 is the most common way to emphasize the suddenness of the idea.

A natural pronunciation would be close to:

갑짜기 조은 생각이 드러써요

A few helpful points:

• 갑자기 → 갑짜기
  The ㅈ often sounds tense here.

• 좋은 → 조은
  The ㅎ is not strongly pronounced in this form.

• 생각이 → 생가기
  The final consonant links into the next syllable.

• 들었어요 → 드러써요
  This is the natural connected pronunciation.

So even though the spelling is 갑자기 좋은 생각이 들었어요, the spoken form sounds smoother and more connected.

The key pattern is:

N이/가 들다

More specifically here:

좋은 생각이 들다

This pattern is used when a thought, feeling, or impression comes into someone’s mind.

Similar examples:

• 이상한 생각이 들어요 = A strange thought comes to mind
• 그런 생각이 들었어요 = I had that thought / That thought occurred to me
• 불안한 느낌이 들어요 = I have an uneasy feeling

So this sentence is a very useful example of a common Korean pattern where something internal comes to mind rather than being expressed as a direct action like I thought.