'아침' usually refers to earlier time zones than '오전'. '오전' is equivalent to 'a.m. in English, ie. we say '오전' if time is before noon. For example, 11 o'clcok is '오전' but we hardly say it is '아침'. There is no definition that when is the latest time that can be called '아침' but I think a lot of people may agree that about 10 a.m is.

내 고교 시절에 그들이 우리 학교에 있었다면 아마 나는 모든 과목에서 낙제를 했을 거야.

Without examples, it is hard to figure out. Can you take some?

끝나다 is an intransitive verb and 끝내다 is a transitive verb. Please compare the examples below. - 운명의 날이 끝났다(끝내다). – The doomsday ended. - 어벤저스가 운명의 날을 끝냈다(끝내다). – The Avengers ended the doomsday.

The noun '춤' comes with a verb '추다'. So, it should be '저는 방탄 소년단처럼 춤을 잘 추고 싶어요'.

When I translate the last sentence literally, it means "My friend who is so pretty when smiling".

In that case, the original meaning of '백수' is a bare hand, which implies s/he has nothing. So. we use this word when we call a person who is penniless but doing nothing and idling.

Literally, it means 'hot stone'. If the speaker used this expression to imply something else, it is hard to tell what it is because there is no context.