The Grammatical aspect reference article from the English Wikipedia on 24-Jul-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Grammatical aspect

For thoughtful child sponsors
Grammatical aspect in linguistics is a property of a verb that defines the temporal flow of the described event or state. The typical contrasts of aspect in many languages can be shown using phrases in English. Here are some of the many aspects found in the world's languages:

'I would/used to walk home from work.' (past habit) In some languages, such as Russian, aspect is more salient than tense in narrative. Russian, like others, marks aspect using special morphology on the verb instead of periphrasis (auxiliaries, adverbs, etc.) as in English. Arabic shows a contrast between dynamic and static aspect (the concepts 'ride' and 'mount' are shown by the same verb, rukubun, static in the former case and dynamic in the latter).

It is important to note that linguistic aspect is distinct from tense, which pinpoints the time at which an action takes place, and is not related to its degree of completion (which might be a good way to describe aspect in layman's terms). Unfortunately, English (which, like most Indo-European languages, hopelessly muddles tense and aspect in its verb system) is not ideal when attempting to underscore this distinction.

English Serbian
I was eating Ja sam jeo
I have eaten Ja sam pojeo
I will eat Ja ću jesti
[I will (intend to) have eaten] Ja ću pojesti
I have been eating Ja sam bio jeo
[I have eaten (a long time ago)] Ja sam bio pojeo
An example will be made of the verb "to eat" in Serbian language. In Serbian, the verb exist in perfective and imperfective aspects; it could be translated either as "jesti" (imperfective) or "pojesti" (perfective). Now, each aspect could be used with each tense of Serbian (except present tense). Notice that, in first two examples, what in English language is expressed in two different tenses, in Serbian is expressed in the same tense, but with two different aspects. The second and third pair of examples show how aspects are combined with other tenses.

English is generally considered to have two tenses, present and past, and these are then modified by two aspects, progessive/continuous and perfect. Tenses are then named according to the combination of aspects they posses. So we have for the present tense:

It is to be stressed that these are the structural expressions of aspect and can convey meanings that would be expressed by separate and different aspects in other languages.

As well as the two tenses, English has a certain number of auxiliary verbs called modals and these are combined with the infinitive to convey a variety of meanings, including those normally expressed in other European languages by the future and conditional tenses:

When combined with the modal auxiliaries the infinitive form changes to accommodate the same combinations of aspect available for the two tenses: In some languages, aspect and tense are very clearly separated, making them more distinct to their speakers.

There are also a number of languages which care much more about aspect than tense. Prominent in this category is Chinese, which differentiates a whole slew of aspects but relies exclusively on (optional) time-words to temporally pinpoint an action.