All you need to have a complete sentence is a subject, a predicate -- a verb -- and a complete thought. Go through these concepts with simple sentences, and then have each student write a complete sentence on a piece of paper. Have them trade papers with a partner, who should mark the subject and predicate or verb. They should also evaluate whether or not the sentence is a complete thought. Evaluating the work of other students will help your learners approach the problem a different way. To add some fun to this activity, you can play a variety of games with the sentences, such as "Telephone." Have this first sentence be the opening of a story. The grading partner marks the sentence, and then adds a sentence of his own that would continue the story, and then passes it on to the next student. The paper continues around the room, a sentence at a time, until the student on the other side of the original writer puts in the closing sentence. The "Telephone" part comes from the fact that each writer only reads the sentence right before the one that she adds. At the end of the activity, each student should have a paper to read, and will have written and checked as many sentences as there are people in the class.
Once your students are comfortable writing complete sentences, have each one write a short story of no more than 10 to 12 sentences. Have them erase eight to 10 nouns, adjectives or verbs, and make a blank. Beneath that blank, they should write the part of speech that they just erased. Then they should turn to a partner and ask the partner for the missing words, by part of speech. For example, the student could say "I need a noun." The first noun that the partner thinks of goes in that blank. The game continues until the student's story is filled up. The writer then reads the completed story to her partner or to the whole class. Next, it's the partner's turn.
In the older elementary grades, students start to learn about complex and compound sentences, in which conjunctions join clauses together to make longer complete thoughts. Make several 8 1/2-by-11 laminated sheets with the words "and," "but," "or," "because," "before" and several other subordinating conjunctions. Then, write enough clauses that each person in the room will have one, and that they work with the conjunctions in the room. Start some music in the room, and give students some time to find the correct partners. You should end up with groups of three -- two clauses with a conjunction. Remember to make several "and" and "but" cards so that you'll have enough conjunctions.
Many different websites have interactive games to help students learn the proper structure of sentences. If you have a day when you can take your students into the computer lab during Language Arts time, you can give the students several options for games to play, or pick one game for all of the students to play, recording high scores on the whiteboard throughout the class period. Have small prizes on hand for the students.