연습장 :)

When I was in Busan I stayed at my friends house
내가 부산에 있었을때 친구집에서 지냈어요
I'm listening to music while I study korean:
나는 음악을 들으면서 한국어를 공부해요
how long have you been studying english?
영어를 얼마동안 공부 했어요?

연습장 :)

When I was in Busan I stayed at my friends house
내가 부산에 갔을때 병조네집에 지냈어요
I'm listening to music while I study korean:
그냥 음악 들으면서 한국어 공부해요
how long have you been studying english?
영어 얼마나 공부 했어요?

=======================================

uuuuuuuuuuuuuuuuuuuuuuuuuummmmmmmmmmmmmmmmm

I was going to go ... not to correct you...

because some corrected your writing but... kkkkkk

When I was in Busan I stayed at my friends house
내가 부산에 갔을때 병조네집에 지냈어요

내가 부산에 있을 때 병조네 집에서 지냈어요

-> the past form of "있을" is "있었을" but you don't need to use "있었을"

It's more natural "있을" even if you want to write in formal form.

I'm listening to music while I study korean:
그냥 음악 들으면서 한국어 공부해요

-> 한국어 공부하면서 음악을 들어요 (it feels like English translation but it's possible)

한국어 공부할 때 음악 들으면서 해요 (it's more natural in Korean)

how long have you been studying english?
영어 얼마나 공부 했어요?

--> you don't need to write "를" 

When I heard "영어를 얼마나 공부했어요?", ummm it's possible and understandable but.. it' not natural..

"영어 얼마동안 공부했어요?" is better expression.

And your original writing is somewhat different nuiance but we can say as the same meaning

"영어 얼마동안 공부했어요?" and "영어 얼마나 공부했어요?" 

연습장 :)

When I was in Busan I stayed at my friends house
내가 부산에 갔을 때 병조네집에서 지냈어요
I'm listening to music while I study korean:
그냥 음악 들으면서 한국어 공부해요
how long have you been studying english?
영어 얼마나 공부 했어요?