안 and 속 both can mean “in” or “inside”, but there are some differences between the two that can be hard to distinguish.

아니다 = to not be, 없다 = not exist, not have, 안 & 지 않다 = not, don’t, 못 & 지 못하다 = can’t do, ~(으)ㄹ 수 없다 = can't, 지 마세요 = don't do it