This is a two page worksheet on the three major sentence errors: comma splices, run-on sentences, and fragments. You could use it as classwork, homework, or even a quiz.
The first side has 15 sentences. Students have to identify the type of sentence error --a comma splice or a run-on-- and then correct it.
On the second side, students do the same, but it deals just with fragments; also, it has 10 sentences.
Give the students the worksheet. Do the first question with them. Give them a few minutes to do the first side. Review the first side. Then, have them do the second side, and review the answers to the second side. This should take about 25-30 minutes, depending on how fast everyone goes.
To make this fill a 50 minute period, have students identify the subject-verb-object (what makes a complete sentence complete) in each phrase while doing the worksheet. Then, have students go to the board to write the corrected sentences, labeling the S-V-O, identifying the type of error, and writing the correction.