To encourage your child to use sentences instead of single words, expand on the phrase your child says. Very often this will be a request for food, pointing out objects, or identifying bodily features. Do not try to correct grammar yet, especially when irregular verbs or tenses are wrong. Rather repeat the sentence correctly, but do not inform your child of the mistakes.
Constant prompting to use a phrase in a full sentence will not encourage your child to do so. Instead, it will interrupt the normal flow of thought and cause frustration. When you read to your child, or watch television, make the experience interactive, by commenting and asking your child questions about what is going on in the story or on the screen.