Making basic mistakes is very easy to do, even in English, so one of the key things to do when writing a Spanish essay is to make a plan. Small but detailed mind maps or bullet points can be so helpful before writing an essay. Simply by writing a few key words or grammar points you will remind yourself of important structures and ideas. Also make sure to read over everything you have written, both as you are writing and at the end. Final checks are essential in highlighting mistakes such as using the plural instead of singular forms.