This can be used as a review or as a primary exercise for teaching students about complete sentences. You could even use this as in-class practice, homework, or even a quiz. I've taught and used this at the middle school, high school, and college levels. Each level will gain and take away a different kind of mastery of this content.
In this worksheet, ten sentences are given where students must identify subjects, verbs, and objects. Once they understand these fundamentals, then they can start understanding sentence errors and how to fix them. (How can they know how to fix a sentence if they don't even understand what a complete/correct sentence is?)
An answer sheet is provided. Depending on your students' level of grammar, times will vary, but when I do this with my classes, it usually goes from 15 to 20 minutes.
Directions: go through the instructions and the example with the students. Then, complete the first item together. Give them a few minutes to go through the worksheet. Then, review each item as a class, explaining things as you go along.