Simple sentences are short sentences, and they are used to express a direct point or for effect. They are often used to develop suspence:
He moved slowly. It was very quiet. The room was dark.
A common way of using short sentences is by constructing introduction sentences for paragraphs.
Smoking causes disease. (introduction sentence) Many people get sick and end up in hospital,because they chose to smoke, and this is very expensive for society.
Long sentences are either complex sentences or complex compound sentences. They are used to express how an idea may be related to other ideas, and are used to give the justification for an introduction sentence in a paragraph.
Smoking causes disease. Many people get sick and end up in hospital,because they chose to smoke, and this is very expensive for society. (detail sentence)
INCORRECT SENTENCE: - He bought the car, it was cheap.