Start with basic explanations and move to error correction
If the student can't identify the problem, mis-identifies some other problem in the sentence as the major problem, or just looks blank, you'll need to start with some basic explanations and work your way toward more complexity over time.
After you've explained the problem, show the student how to edit to correct the first sentence. Ask the student to identify the error and edit the second example of the same type of error. (If the student still can't fix the error, try another variation of your explanation and work with more examples.) As soon as the student can fix two or three errors in a row, have her fix one more with you watching. Then ask her to fix the remaining sentences with the same type of error in the two-page sample you marked. If the student can do those without your intervention, have the student look for additional errors of the same type in the next page or so of the text.
When the student is confident of both finding and fixing errors of this sub-type, move to the next sub-type.