V-다가 grammar = and/but then ~express interruption of one action to do another

V-다가 grammar = and/but then ~express interruption of one action to do another 

Usage:

(1) Express the speaker’s interruption of one action to do another either intentionally or unintentionally = (while)... and then, but then

아까 낮에 뭐 했니?
- 만화책을 읽다가 친구하고 같이 외출했어. (intentionally)

What did you do this afternoon?
- I was reading a comic book, but then I went out with a friend.

뛰어 가다가 계단에서 미그러졌어요. (unintentionally)

While running, I slipped on the stairs.

(2) The grammar can be also used to indicate the continuation of the action in the 1st clause while the action in the 2nd clause occurs. 

친구를 이야기를 하다가 웃었어요.

I was talking to my friend and laughed.

(3) The grammar can also be used with some adjectives to indicate the change from one state to another. 

날씨가 맑다가 갑자기 흐려졌어요.

It was sunny and suddenly became cloudy.

(4) Both clauses must have the same subject. 

밥을 먹다가 전화를 받았습니다. (Correct) 

밥을 먹다가 전화가 왔어요. (Wrong)

I was eating and then answered the phone.

- V-아/어다가 grammar = and then ~action is done as a result of the previous action 

- A/V-다가는 grammar = if (the action is continued), a negative outcome will occur

- V-았/었다가 grammar = but then ~after the completion of an event, the opposite event happens

Verb +-다가: 

보다 -- >보다가 (I was watching and then...)
먹다 --> 먹다가  (I was eating and then...)
잠을 자다 --> 잠을 자다가 (I was sleeping and then...)
읽다 --> 읽다가 (I was reading and then...)
일하다 --> 일다가 (I was working and then...)
나가다 --> 나가다가 (I was going out and then...)
이야기하다 --> 이야기하다가 (I was talking and then...)

Examples:

1. 집에 가다가 친구를 만났어요.

I was going home and I met a friend.

2. 공부를 하다가 졸려서 잤어요.

I was studying and then dozed off, so I slept.

3 지하철 1호선을 타고 가다가 시청역에서 2호선을 갈아타세요.

Take the subway line 1, and then change to line 2 at City Hall station.

4. 축구를 하다가 넘어져서 다쳤어요.

I was playing soccer and then I felt and got hurt.

5. 잠을 자다가 친구에게 전화가 와서 나갔어요.

I was sleeping and then I got a call from a friend, so I went out.

6. 한국에서 대학교를 다니다가 그녀를 만났어요.

While I went to university in Korea, I met her.

7. 아이가 왜 다쳤어요?
- 야구를 하다가 공에 맞았어요.

Why did the kid get hurt?
- He was playing baseball and then got hit by a ball.

8. 어디에 다녀오세요?
- 요리를 하다가 파가 없어서 사 왔어요.

Where are you going?
- I was cooking, but then there was no onion, so I went out to buy some.

9. 잠을 자다가 무서운 꿈을 꿨어요.

I was sleeping and had a scary dream.

10. 영화를 보다가 잠들었어요. 
While watching a movie, I fell asleep.

11. 중국어를 배우다가 너무 어려워서 포기했어요.
While learning Chinese, I gave up because it was too difficult.

12. 미국에서 회사에 다니다가 한국으로 이사왔어요.
I've worked for a company in America and then I moved in Korea.

13. 밖이 조용하다가 점점 시끄러워졌어요.

It was quiet outside, but gradually became noisy.

14. 양파를 썰다가 손가락을 다쳤어요.

While cutting onions, I hurt my finger.

15. 밥을 먹다가 실수로 혀를 깨물었어요. 

While eating, I accidentally bit my tongue.

16. 아이가 놀이터에서 놀다가 넘어졌어요.

The child fell while playing in the playground.

17. 좋은 일이 있어요? 왜 그렇게 웃어요?

- 길을 가다가 5만원을 주웠어요.

Do you have any good news? Why are you laughing like that?

- While going on the street, I picked up 50,000 won.