- 하던 일을 마저 해 치우자.
- 한 일을 마저 해 치우자.
Sentence 1 is the correct sentence.
하던 일을 마저 해 치우자.
Let's finish the work we were doing.
Sentence 1 is correct because 던 can imply that the work was interrupted, whereas 한 implies that the work was completed, leaving nothing to finish.
Now let's consider the opposite situation. Which of the following two sentences would be correct?
- 여기가 내가 졸업하던 학교다.
- 여기가 내가 졸업한 학교다.
Sentence 2 is correct.
여기가 내가 졸업한 학교다.
This is the school I graduated from.
Sentence 2 is correct because the verb 졸업하다 means you "have completed your studies," so 한 is more appropriate here than 하던.