“Errors may result from unclear thought or from ignorance. Those that result from unclear thought constitute a large proportion of all sorts that history students make. If the underlying thought in an essay is confused, the confusion will work through into the choice of words, the grammar, the punctuation, and all aspects of the style.
It is for this reason that, in drafting, you need to examine every sentence that gives you the slightest qualm, asking yourself exactly what is the thought behind it. What are the things about which the sentence is talking, and what exactly are the relationships between those things? What sorts of things are they anyway? – people in the past, or things written a long time ago about people in the past, or books about the things written about people in the past? Sometimes, when you can get these categories and relationships straight, the problems of style and grammar may solve themselves.
Thus it happens very often that a student will write clumsy and ill-expressed sentences, with various errors of English in them, not really because of any intractable ignorance of the principles of English expression, but simply because of confusion about subject matter."
I.W. Mabbett, Writing History Essays: A Student’s Guide (Palgrave Macmillan, 2006), 101.