전복 has two different meanings. First of all, it is a sea food, yes. Second of all, it is overturn or overtrhow :)

구전 means keep telling the story by mouth to mouth for a long time ago and this is noun form. 중요 is noun form of 중요하다. Simply importance

"는 곳(은)" means pointing some place and it will be changed by which verbs are in front of 는 곳. And this could be noun form. For example 사는 곳 : living place 파는 곳: being sold something(=a store) 먹는 곳: be able to eat something(=식당, restaurant) And -인가(요) means ask something or have some question For example, 이것은 사람인가요? : Is this a human being??? 이것은 짐승인가, 사람인가? : Is this a human being or a beast? 당신은 선생님인가요? Are you a teacher? -인가(요) is simply defined 입니까