He’s straightforward. (Communicates honestly and efficiently. Positive) He’s simple. (Unintelligent. Negative) There’s a simple trick to getting this to work. (Not ‘straightforward’) A scientific explanation of where rainbows come from can be straightforward, but still not simple enough for children to understand.

straightforward - direct simple - clear / comprehensible

I agree with Spangola, they are the same. This is a simple recipe to make chocolate cake. This is a straightforward recipe to make chocolate cake.

straightforward means you just directly talk what you think, no need to talk around. simple means uncomplicated, no difficulty.

There isn't any. They both mean "uncomplicated."