Learn Hardcore Korean: geuneun honja gominhadaga gyeolguk haegyeolhaesseo. - He agonized alone and eventually solved it.

-다가 attaches to a verb stem to show that while doing A, a new event B happened or there was a shift from A to B.
Here: He was in the middle of 고민하다 and then the event 해결했어 occurred.
It often suggests a natural progression or turning point.

How is -다가 different from -고?

-고 links actions neutrally in sequence or lists them: A and B.
-다가 highlights that B happened during or after A as a development, often implying A ceased or gave way to B.
Compare: 혼자 고민하고 결국 해결했어 (plain sequence) vs 혼자 고민하다가 결국 해결했어 (solving emerged from the state of worrying).

Is 그는 natural in everyday conversation?

In casual speech, Korean usually drops third-person pronouns; 그는 is more common in writing.
More natural in speech:
- Omit the subject: 혼자 고민하다가 결국 해결했어.
- Or use 걔는/걔가 (familiar), 그 사람은/그분은 (neutral/polite), or a name/title.

Why use the topic marker -는? Could I say 그가 instead?

그는 sets him as the topic and can imply contrast or continuation of a known topic.
그가 marks him as the grammatical subject, often presenting new information.
Both work. For a simple who-did-it report, 그가 is fine; to continue talking about him or contrast with others, 그는 is better.

What exactly does 혼자 mean here? How is it different from 혼자서 and 스스로?

혼자: alone; no one else is with or involved.
혼자서: stronger emphasis on by oneself; often interchangeable with 혼자.
스스로: by one’s own effort/initiative (even if others are around). 스스로 해결했어 stresses no help was needed.

Does 고민하다 mean the same as 걱정하다?

고민하다: to agonize or deliberate over a problem/choice; mental struggle to figure things out.
걱정하다: to worry or feel anxious about something.
고민하다 fits naturally with 해결하다.

Why is there no object after 해결했어? What is being solved?

Korean often drops objects when context makes them obvious. The implied object is 문제, 일, 상황, etc.
Explicit versions: 문제를 해결했어, 상황을 해결했어, 문제를 스스로 해결했어, and so on.

What politeness level is 해결했어, and how do I make it polite or honorific?

Where can 결국 go? Is 결국에 acceptable?

Natural placements: 그는 결국 해결했어, 그는 혼자 고민하다가 결국 해결했어, or 결국 그는… for emphasis.
결국에 is colloquial and less standard; 결국에는 is fine when adding emphasis: 결국에는 해결했어.

Could I use -더니 instead of -다가?

Yes, with nuance change: 그는 혼자 고민하더니 결국 해결했어.
- -더니 links an earlier observed state to a later result and implies the speaker witnessed the first part.
- -다가 is more neutral about observation and simply marks the transition.

What is the nuance of 결국 compared to 드디어 or 마침내?

결국: in the end, as a result; neutral and can be used for positive or negative outcomes.
드디어: finally, at last, with a long-awaited, positive feel.
마침내: finally, somewhat literary/dramatic. All can fit, but the tone differs.

Is there a difference between 해결했어 and alternatives like 해결해냈어 or 풀었어?

해결했어: solved/resolved (general).
해결해냈어: managed to solve (emphasizes effort and achievement).
풀었어: solved, typically for problems/riddles/equations. For situations or conflicts, 해결하다 is more natural.

What’s happening morphologically in 해결했어?

Any pronunciation tips for the whole sentence?

고민하다가 is [고미나다가], 결국 is [결국], 해결했어 is close to [해결해써] (the ㅆ like s in see).
Keep the flow smooth across -다가 into the next clause.

Is the word order flexible?

Yes, within reason:
- Most natural: 그는 혼자 고민하다가 결국 해결했어.
- Also okay: 그는 결국 혼자 고민하다가 해결했어, 결국 그는 혼자 고민하다가 해결했어.
- Avoid separating 혼자 far from 고민하다가.

Do I need a comma before 결국?

No. Punctuation is optional for rhythm in longer sentences, but 그는 혼자 고민하다가 결국 해결했어 is fine without a comma.

Any constraints or pitfalls with -다가?

The subject of both clauses is generally the same.
It often implies the first action stops or yields to the second.
For clear cause, use -아서/어서; for simple sequence, -고; for observed correlation, -더니.

How do I emphasize that he solved it by himself, not just that he worried alone?

Is the spacing correct? Should it be 고민 하다가?

Can the sentence end after -다가?

No. -다가 is a connective and must be followed by a main clause.
Ending after it would be incomplete. A different pattern like -다가 말았어 exists but changes the meaning to he stopped while doing it.

Can I say 그녀는 instead of 그는?

Grammatically yes, but in everyday Korean 그녀 and 그 are uncommon. Prefer a name/title, 걔, 그 사람, or simply omit the subject unless needed for clarity.