Simple-Easily understood or done; presenting no difficulty.E.g Simple task Plain-Not decorated or elaborate; simple or ordinary in character.E.g Plain cake

While "simple" often carries a meaning of uneducated or unintelligent, I often describe myself as a "simple person". I do not like to go to fancy places. I do not like getting dressed up (except if I am acting in a stage play). My tastes are simple. I like to eat raw fruit. I would rather walk than ride in a limo. I take the direct approach to problem solving. For me, an "elegant" solution is one that is straightforward and simple.

Do you have any examples? The meanings can change a lot, e.g.: She is very plain = unexceptional, ordinary She is very simple = uneducated, unintelligent It's simple = easy, not complicated It's plain (to see) = obvious, clear ...and many more. It really depends on the sentence...