The issue I see most often in student writing is the use of a pronoun which does not refer to one specific noun.
To review, a pronoun is a word used to take the place of a noun and should refer to one unmistakable noun preceding it. This noun is called the pronoun’s antecedent. Unfortunately, it is very easy to create a sentence that uses a pronoun without a clear antecedent.
Take a look at this example:
“After putting the disk in the cabinet, Mabel sold it.”