Beginning a sentence with the imperative form of a verb may feel inappropriate, as it essentially involves telling the reader what to do. However, in some cases an imperative can further engage readers, enabling them to take a more active role in understanding your essay. Sentences that begin with verbs such as "envision," "understand" and "compare" ask readers to use their powers of imagination and deductive reasoning. Imperative sentences are also common in instructional writing, which may ask the reader to "Describe the physical properties of Substance A" or to "Save your document as a PDF."
An essay is an exercise in rhetoric, or the art of presenting information in a persuasive manner. Rhetorical questions can make readers think about the topic at hand. Such questions follow the structure of the interrogative, but they do not allow the reader to answer directly. Instead, you should provide an answer immediately following the rhetorical question. For example, "Is Miss Havisham a sympathetic figure? Her ability to recognize and repent her mistakes certainly implies that she has a moral center of sorts."
The infinitive is the root form of the verb, such as "to be," "to run" or "to understand." Sentences that begin with infinitive phrases -- such as "To create a truly democratic society, leaders must establish an infrastructure that allows for universal suffrage" -- indicate that the second phrase is a necessary step in accomplishing the first.
Linking verbs, such as "have," "be," and "shall," can be used as auxiliary verbs, which indicate the context of the main clause. For example, "Had Edison known the danger X-rays posed, he may have exercised more caution when experimenting on human subjects." In some cases, beginning a sentence with an auxiliary verb also indicates the time frame and tense of the other clause: "Should the filibuster extend past midnight, the senators may not vote on the bill."