In some languages, aspect and time are very clearly separated, making them much more distinct to their speakers. There are a number of languages that mark aspect much more saliently than time. Prominent in this category is Chinese, which differentiates many aspects but relies exclusively on (optional) time-words to pinpoint an action with respect to time. In other language groups, for example in most modern Indo-European languages (except Slavic languages), aspect has become almost entirely conflated, in the tense system, with time.
In Russian, aspect is more salient than tense in narrative. Russian, like other Slavic languages, uses different lexical entries for the different aspects, whereas other languages mark them morphologically, and still others with auxiliaries (e.g., English).
In high Arabic (الفصحى, "al-Fusha", the clear (language)) the verb is marked for tense but not aspect, according to the indigenous tradition. The "Past Verb" (فعل ماضي, fi'l maadiy) denotes an event (حدث, hadath) completed in the past, but says nothing about the relation of this past event to present status, nor about aspect, at least not directly. For example, "وصل", wasala, "he arrived", indicates that arrival occurred in the past without saying anything about the present status of the arriver - maybe he stuck around, maybe he turned around and left, etc. - nor about the aspect of the past event except insofar as completeness can be considered aspectual. This "Past Verb" is clearly similar if not identical to the Greek Aorist, which is considered a tense but is more of an aspect marker. In the Arabic, aorist aspect is the logical consequence of past tense. By contrast, the "Verb of Similarity" (فعل المضارعة, fi'l al-mudaara'ah), so called because of its resemblance to the active participial noun, is considered to denote an event in the present or future without committing to a specific aspectual sense beyond the incompleteness implied by the tense: يضرب "yadribu", he strikes/is striking/will strike/etc. Those are the only two "tenses" in Arabic (not counting "أمر"، "amr", command, which the tradition counts as denoting future events.) At least that's the way the tradition sees it. To explicitly mark aspect, Arabic uses a variety of lexical and syntactic devices.
Contemporary Arabic dialects are another matter. One major change from al-Fusha is the use of a prefix particle (ب "bi" in most dialects) to explicitly mark progressive, continuous, or habitual aspect: بيكتب, bi-yiktib, he is now writing, writes all the time, etc.
Aspect can mark the stage of an action. The inchoative identifies that the action is soon to take place. The inceptive aspect identifies the beginning stage of an action (e.g. Esperanto uses ek-, e.g. Mi ekmanĝas, "I am beginning to eat."). Aspects of stage continue through progressive, pausative, resumptive, cessive, and terminative.
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.
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.