Important qualifications:

Although the perfective is often thought of as representing a "momentary action", this is not strictly correct. It can equally well be used for an action that took time, as long as it is conceived of as a unit, with a clearly defined start and end, such as "Last summer I visited France".

Grammatical aspect represents a formal distinction encoded in the grammar of a language. Although languages that are described as having imperfective and perfective aspects will agree in most cases in their usage of these aspects, no two languages will agree in every situation. For example:

Some languages have additional grammatical aspects. Spanish and Ancient Greek, for example, have a perfect aspect (not the same as the perfective), which refers to a state resulting from a previous action (also described as a previous action with relevance to a particular time, or a previous action viewed from the perspective of a later time). This corresponds (roughly) to the "have X-ed" construction in English, as in "I have recently eaten". Languages that lack this aspect (such as Portuguese, which is closely related to Spanish) often use the past perfective to render the present perfect (compare the roughly synonymous English sentences "Have you eaten yet?" and "Did you eat yet?").

In some languages, the formal representation of aspect is optional, and can be omitted when the aspect is clear from context or does not need to be emphasized. This is the case, for example, in Mandarin Chinese, with the perfective suffix le and (especially) the imperfective zhe.

For some verbs in some languages, the difference between perfective and imperfective conveys an additional meaning difference; in such cases, the two aspects will typically be translated using separate verbs in English. In Greek, for example, the imperfective sometimes adds the notion of "try to do something" (the so-called conative imperfect); hence the same verb, in the imperfective (present or imperfect tense) and aorist, respectively, is used to convey look and see, search and find, listen and hear. (For example, ηκουομεν ēkouomen "we listened" vs. ηκουσαμεν ēkousamen "we heard".) Spanish has similar pairs for certain verbs, such as (imperfect and preterite, respectively) sabía "I knew" vs. supe "I found out", podía "I was able to" vs. pude "I succeeded (in doing something)", quería "I wanted to" vs. quise "I tried to", no quería "I did not want to" vs. no quise "I refused (to do something)". Such differences are often highly language-specific.

English

According to one prevalent account, the English tense system has only two basic tenses, present and past. No primitive future tense exists in English; the futurity of an event is expressed through the use of the auxiliary verbs "will" and "shall", by use of a present form, as in "tomorrow we go to Newark", or by some other means. Present and past, in contrast, can be expressed using direct modifications of the verb, which may be modified further by the progressive aspect (also called the continuous aspect), the perfect aspect (also called the completed aspect), or both. Each tense is named according to its combination of aspects and time. These two aspects are also referred to as BE + ING (for the first) and as HAVE +EN (for the second). Although a little unwieldy, such tags allow us to avoid the suggestion that uses of the aspect BE + ING always have a "progressive" or "continuous" meaning, which they do not.

For the present tense:

Present Simple (not progressive/continuous, not perfect; simple): "I eat"

Present Progressive (progressive, not perfect): "I am eating"

Present Perfect (not progressive, perfect): "I have eaten"

Present Perfect Progressive (progressive, perfect): "I have been eating"

For the past tense:

Past Simple (not progressive/continuous, not perfect; simple): "I ate"

Past Progressive (progressive, not perfect): "I was eating"

Past Perfect (not progressive, perfect): "I had eaten"

Past Perfect Progressive (progressive, perfect): "I had been eating"

(Note that, while many elementary discussions of English grammar would classify the Present Perfect as a past tense, from the standpoint of strict linguistics – and that elucidated here – it is clearly a species of the present, as we cannot say of someone now deceased that he "has eaten" or "has been eating"; the present auxiliary implies that he is in some way present (alive), even if the action denoted is completed (perfect) or partially completed (progressive perfect).)

The uses of these two aspects are quite complex. They may refer to the viewpoint of the speaker:

I was walking down the road when I met Michael Jackson's lawyer. (Speaker viewpoint in middle of action)

I have travelled widely, but I have never been to Moscow. (Speaker viewpoint at end of action)

But they can have other meanings:

You are being stupid now. (You are doing it deliberately)

You are not having chocolate with your sausages! (I forbid it)

I am having lunch with Mike tomorrow. (It is decided)

Another aspect that does survive in English, but that is no longer productive, is the frequentative, which conveys the sense of continuously repeated action; while prominent in Latin, it is omitted from most discussions of English grammar, as it suggests itself only by Scandinavian suffixes no longer heard independently from the words to which they are affixed (e.g., "blabber" for "blab", "chatter" for "chat", "dribble" for "drip", "crackle" for "crack", etc.).

Note that the aspectual systems of certain dialects of English, such as Hawaiian Creole English and African-American Vernacular English, are quite different from standard English, and often distinguish aspect at the expense of tense.

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Slavic languages

In Slavic languages there is only one type of aspectual opposition which forms two grammatical aspects: perfective and imperfective (in contrast with English which has two aspectual oppositions: perfect vs. neutral and progressive vs. nonprogressive). The aspectual distinctions exist on the lexical level - there is no unique method to form a perfective verb from a given imperfective one (or conversely). Perfective verbs may be formed by means of prefixes, changes in the root, or using a completely different root (suppletion). Note, however, that possessing a prefix does not necessarily mean that a verb is perfective.

With a few exceptions each Slavic verb is either perfective or imperfective. Most verbs form strict pairs of one perfective and one imperfective verb with generally the same meaning. However, each Slavic language contains a number of verbs which are bi-aspectual and act as both imperfective and perfective. They are mainly borrowings from non-Slavic languages, but some native verbs also belong to this group. As opposed to them, mono-aspectual verbs are mainly native. There are mono-aspectual imperfective verbs without perfective equivalents (among others, verbs with the meaning 'to be' and 'to have') as well as perfective verbs without imperfective equivalents (for instance, verbs with the meaning 'become ...', e.g. 'to become paralyzed', etc.).

The perfective aspect allows the speaker to describe the action as finished, completed, finished in the natural way. The imperfective aspect does not present the action as finished, but rather as pending or ongoing.

An example is the verb 'to eat' in the Serbo-Croatian language. The verb translates either as jesti (imperfective) or pojesti (perfective). Now, both aspects could be used in the same tense of Serbian. For example (omitting, for simplicity, feminine forms like jela):Serbo-Croatian

Example Tense Aspect

Ja sam jeo/ Ja сaм јеo past imperfective

Ja sam pojeo/ Ja сaм појеo perfective

Ja sam bio jeo/ Ja сaм био jeo pluperfect imperfective

Ja sam bio pojeo/ Ja сaм био појеo perfective

Ja ću jesti/ Ja ћy jecти future imperfective

Ja ću pojesti/ Ja ћy пojecти perfective

Ja sam pojeo signals that the action was completed. Its meaning can be given as "I ate (something) and I finished eating (it)"; or "I ate (something) up".

Ja sam jeo signals that the action took place (at a specified moment, or in the course of one's life, or every day, etc.); it may mean "I was eating", "I ate" or "I have been eating".

The following examples are from Polish.