A simple sentence is not a plain or short sentence. It is a sentence that conveys only one main idea. Feeling the disappointment of our friends at our early departure, we were sorry to leave before meeting all the guests. Francis ate peanuts and drank coffee. A simple sentence is the most effective way to deliver one main point.
Compound sentences use coordination to connect two ideas of equal importance.
The bus was very crowded, so I had to stand all the way. Avoid long series of compound sentences, since they do not show the relative importance of ideas. Because the bus was crowded, I had to stand all the way. Complex sentences use subordination to show the relative importance of ideas in a sentence. He arrived before we left.