Teachers observe that some students apparently think that formal writing (essays, etc.) is made up of fancy, long words and complex, rambling sentences. This is far from the truth.
Although you may be writing abut complex issues and concepts, and you may sometimes need to use specialised terminology, this does not mean that you write in an overly complex way. In fact, simplicity in writing is the most challenging task. You need to know your subject area very well before you can write clearly and simply, to make it easier for the reader to follow your line of thoughts or arguments.
Make sure you re-read your writing, and if it's too long, with multiple clauses, it may be best if you separate these into separate sentences, especially if you are struggling with sentence structure – the longer the sentence, the easier it is to get the structure wrong.
(Partially adapted from The University of Hull)