Come here, James. (<~~a complete sentence; a command)
When James came home. (<~~an incomplete sentence)
James came home last night. (<~~a complete sentence)
Students should be able to recognize these and others by the time they exit fourth grade, but far too many cannot. If they cannot recognize them in print or in their own handwriting, how will they ever learn to write in complete sentences? This is definitely something on which parents can work with their children. And here are some websites that can help:
Sentences can be classified in two different ways: by their structure and by their function. First, I'll focus on sentences as defined by their functions.
Most commonly used are declarative sentences, those sentences that make a statement of some kind:
If parents want to help their children gain proficiency in this area, one thing they can do is go to English Zone and click on Grammar Blast - Grade 4. In this area there are various quizzes, at least three of which deal with children's ability to recognize complete sentences.