An imperative sentence is a sentence that issues a command. These sentences generally start with a verb and lack a stated subject. The subject, however, is always the second person "you" and is therefore understood without being explicitly stated.
Three things are common to all imperative sentences. First, they all command someone to do something. Second, they all use an unstated "you," also called the "you understood," as their subject. Third, they all start with a verb or the word "please" followed by a verb.
Commands are generally issued to a person or people who are present. For example, one would not say "wash your hands" to someone they were not speaking to directly. If a person wished to tell someone not present to wash their hands, he or she might inform someone nearby "tell him to wash his hands." The command then changes from "wash" to "tell" and is still issued to a present individual.
Imperative sentences cannot have a subject other than "you." Although the subject is normally omitted from an imperative sentence, "you" can be placed in front of the sentence, and it will still make sense. For example, "you pick up those clothes." Another pronoun cannot be placed in front of the verb and have the sentence retain the same meaning. Saying, "he pick up those clothes" is grammatically incorrect since the correct verb form for singular pronouns is "picks" not "pick."
Replacing "he" with "they" still does not result in a command, however. Although, "they pick up their clothes" is a grammatically correct sentence, its meaning is no longer a command. Instead, this sentence states what "they" are doing rather than tells "them" to do it.
An imperative sentence is always in present tense because commands are issued in the now, not in the past or the future. For example, changing "go" in the command "go to the store" into the past or future tense makes the sentence confusing and grammatically incorrect. "Went to the store" or "will go to the store" leaves the reader wanting to know the "who," or the subject, of the sentence. These sentences are no longer commands.
Although they are most popularly found in speech, imperative sentences do have their place in writing as well. In fiction, they can be found in dialogue. Most often, however, readers will come across an imperative sentence in instructions. Recipes or step-by-step "how to's" use commands to issue directions. In most instructions, in fact, every step begins with an imperative sentence.