Use short sentences to depict fast, tense action or conversations. This establishes the pace. Longer ones work well for descriptive narrative, or more casual dialog. It helps to think about the tone of voice you or your narrator would be using for a given scene, then capture it in the sentence structure itself.

Over all, vary sentence length to maintain reader attention rather than droning on and on…You don’t want your story to come off as being told by that teacher or professor who used to put you to sleep.