You aren't writing in proper sentences....
This is a common criticism of student's essays. Lots of tutors have definite ideas of what is a proper sentence and what isn't. Other comments might be:

• This is badly written...
• This doesn't flow from one idea to the next...
• This doesn't make sense.

It all adds up to the fact that if you don't use the right kind of sentences, then lots of tutors are going to think that you are writing badly. Not writing proper sentences is a serious fault. It's like getting apostrophes wrong, or spelling a word wrong in the title of the essay. You can lose marks for it.

In this section we're going to look at some common mistakes in sentences, and work out ways of spotting them and putting them right.