Prescriptive essays express commands. The essayist writes in the present tense and the imperative mood, using short sentences with active verbs such as "Capitalize the first word," or "Use this format."
The writer often uses the pronoun "you" as the subject of sentences, as in "you must" or "you should." The essay provides explanations in a step-by-step fashion, rarely digressing from the process unless warning the reader about what not to do and why.
Unlike the rule-based format of prescriptive essays, descriptive essays describe how something works, or how people do things, without implying judgment of the rightness or wrongness of specific methods. A descriptive essayist writes in the third person.