웬 or 왠 are so confusing even to Koreans. 웬 means ‘what happened’ and it is used with a lot of words or phrases such as 웬일, 웬만하다, 웬만큼, 웬 떡. However 왠 is only used for 왠지which is the short from for 왜인지.
웬일이에요? What brought you here?
제 친구는 코가 높아서 웬만한 남자는 눈에 안 차요. My friend is very picky, so she is not satisfied with normal guys.
웬만하면 참으세요. If it is okay, please bear with it.
공부를 웬만큼 잘해서는 서울대학교에 갈 수 없어요. You cannot go to Seoul National University if your score is just okay.
그 사람은 웬만해서는 잘 안 웃어요. That person doesn’t usually smile.
웬떡이니? How lucky am I!
On the other hand, 왠지 is the short form for 왜인지 which is indirect question sentence ,so it means ‘I don’t know why’. (You can learn indirect question by clicking here.)
오늘은 왠지 기분이 안 좋아요. I don’t know why but I am not a good mood today.
왠지 안 좋은 일이 생길 것 같아요. I don’t know why but I feel that something bad will happen.
왠지 좀 걷고 싶어요. I don’t know why but I want to walk a little bit.
왠지 좀 추운데 감기에 걸렸나? I don’t know why but I am little bit cold. Do I have a cold?