To put it simply, Dan didn't want to come.
→간단히 설명하자면,댄은 오고 싶어하지 않아.

※ to put it simply=to put it briefly.

〔예문〕
To put it simply, Dan didn't want to come.
간단히 말하자면, 댄은 오고싶어하지 않아

To put it briefly, Rachel is moving,so I need to find a new roommate.
간단히 설명하자면, 레이첼이 이사를 해서 나는 새 룸메이트를 찾아야해.