Explain to the child that all sentences must contain certain components to make sense. For example, a subject, or noun; an object, or the person or thing that receives or is affected by the action; and a predicate, which contains the verb.
Ask the child to write a sentence, or write one for him. Ask questions about the sentence. For example, ask whom the sentence is about and what the subject did or where it went. Such questions help him to become aware of the sentence components and to build a more interesting and informative sentence.
Encourage the student to use action verbs and adjectives. Such verbs describe an action in the sentence, and adjectives add detail and description. Action verbs include such words as "hit," "run" and "jump," and adjectives include words such as "pretty" and "sweet." Including these components makes sentences more complete and informative.
Ask the child to read the sentence out loud and pay attention to where he breathes. If he runs out of breath, he is more likely to have written a run-on sentence. In that case, help him shorten the sentence and show how to make the run-on sentence into two sentences.