Careful use of fragments can add meaning and power to your writing. For example: I was absolutely happy. For now. Those last two words are a fragment but they raise a question or produce doubt. The fragment is also closely connected to the full sentence it follows.
In the following paragraph, notice the fragments, and yet they make sense:
They missed the people from whom they had fled. They missed the sight, the sound, even the smell of Africans. The sing-song chanting of workers in the fields. The tangy smell of wood smoke in the early evening.The fragment also works well (when used cautiously) in action scenes.
In what follows Jason is running his first marathon: He pushed past the first group of runners and was only a dozen yards behind the two leaders. He pushed forward. Too fast. He had to control his pace to win the race. Easy. Easy now. Sneak up behind them.I used fragments. The first is Too fast. I could have written: It was too fast or he moved too quickly. But the purpose is to make readers feel as if they're running with Jason and are inside him. The same for Easy. Easy now. Sneak up behind them.
It's all right to use sentence fragments occasionally
—as long as the meaning is clear.