The imperfective aspect is a grammatical aspect. It refers to an action that is viewed from a particular viewpoint as ongoing, habitual, repeated, or generally containing internal structure.

The opposite usage is the perfective aspect, which views an action as a simple whole (and is not the same as the perfect aspect). In narrative, the imperfective is usually used to describe the background situation ("It was midnight. The room was dark. The rain was beating down. Water streamed in through a broken window. A gun lay on the table."), while the perfective describes actions ("Suddenly, a man burst into the room, ran over to the table, and grabbed the gun.").

English does not have a grammatical form that corresponds exactly to the imperfective aspect. The "progressive tenses", which denote tense with the continuous and progressive aspects, are often used to render the imperfective when it describes an ongoing action (ex: "The rain was beating down").

Another example of the imperfect aspect is when past habitual actions are rendered using "used to", followed by a verb. For example, "I used to ski".

The simple past can substitute for either case without a significant change in meaning; for example, "The rain beat down" in place of "The rain was beating down", and "I walked to school every day" in place of "I used to walk to school every day".

Furthermore, the simple past is almost invariably used to render the imperfective with inherently stative verbs ("was", "had"), and quite often with verbs used in a stative sense ("lay" in the above narrative).

The imperfective aspect is an aspectual component of tenses in various languages, such as Greek, Latin, the Romance languages, and Persian known as the imperfect tense. (In some languages, the "imperfect tense" has a different meaning.)

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