Mr.Bascom's mouth dropped open.

해석은 베스콤씨의 입이 벌어졌다. 인거같아요... 감으로라도 해석이 되는데...

문제는 dropped open. Open은 동사같고 dropped는 분사인데 dropped open 이란 숙어도 없고요..
Dropped를 직역하면 떨어진 베스콤씨의  입이 열렸다 인데 이게 무슨 뜻이죠?? ㅠㅠ

그리고

Success isn't happened by accident.

이문장에서 틀린부분은 happen은 자동사여서 수동형이 안되서 틀린건데..
해설을 보니까 저랑 똑같이 말을 하는데 isn't happened 를 doesn't happened 라고 바꿔야 한다네요?

이거 모죠?? 해설이 틀린거죠?? happen 이 동사인데... 부정은 해주려면 doesn't happen 이 되야하는거 아닌가요??