I have a Language Proficieny Test where I need to explain the students' errors by filling in the blanks after correcting the errors in the first part. The questions are like:

There are two problems in this item. The first problem concerns the incorrect use of (a)the (main) verb "is". the writer should use (b) the simple past tense to indicate the correct time of action.
the second problem concerns a wrong use of (c) the adverb"scarily". the (d) adjective "scary" should be used instead.

I know how to correct the errors but when it comes to explaining the errors (by filling in the blanks like above, it is difficult for me. Can anyone suggest some practice online with some guidelines?