간단하다 = simple and short (implies short and easy). 단순하다 = straightforward without unexpected complication. (may be used for things involving a long process) 간단하다 thus implies a task, process, or principle that can be finished in a few simple steps. 단순하다 may be used for more substantial things, such as a manufacturing process, if every part of it is simple. It can be used for people too, as in 그는 단순한 (simple-minded) 사람이다, whereas 그는 간단한 사람이다 is unnatural because people are fundamentally complex beings.