Most of the teachers here presume that you have at least a good grasp of the rules of grammar. That is the baseline because the tight schedule forbids us to teach you the basic rules of grammar again. But if you are determined to brush up your grammar, then reading the articles you are assigned to is a good starting point. It may take a while but the process is rewarding. It is not surprising to note that most of the good essays are written in simple English.
Simplicity implies precision and accuracy.
In other words, your essay must not contain an overwhelmingly high proportion of wrong grammatical uses. In this respect, a standard dictionary can help you solve the problem. Never, at least at this stage of technological development, trust the grammar check of your computer because it is so rudimentary. If we have now got over with the grammar burden, we have to consider the problem of style. There are two senses of style.