Review your essay carefully, very slowly, sentence by sentence for subject/verb agreement, tense sequence, plurals and possessives, sentence fragments, and run-on sentences. Once you have checked all of these things, check again; they are very easy to miss.
The following are some particularly important questions to consider with respect to grammar in your essay:
Does each subject agree with its verb (e.g., “the girl works”; “the girls work”)?
Is the tense of your verbs appropriate and consistent within each paragraph?
Can each pronoun (he, she, they, it, etc.) be easily traced back to the person or thing to which it refers?
Are all your sentences complete and appropriate in length?
Does each sentence have a subject and a verb?
If sentences are combined, have you used a conjunction or a semi-colon between them?