English verb have special forms for expressing actions in progress, going on at a definite moment or period of time, i.e. for expressing limited duration - continuous forms.
Continuous forms have been traditionally treated as tense forms (definite, expanded, progressive) or as tense-aspect forms.
Members of the opposition are not opposed as tenses (tense is the same). They show different character of an action, the way in which the action is experienced or regarded: as a mere fact or as taken in progress. The opposition common-continuous reveals the category of aspect.
Tense and aspect are ciosely connected, but they are different categories, revealed through different oppositions: comes — came; comes — is coming.
The fact that the Infinitive has the category of aspect (to come — to be coming) and has no category of tense also shows, that these are different categories.
The category of _asgect is closely connected with the lexical meaning. R.Quirk divides the verbs into dynamic (having the category of aspect) and stative (disallowing the continuous form). Stative verbs denote perception, cognition and certain relations: see, know, like, belong. Dynamic verbs may be terminative (limitive), denoting actions of limited duration: close, break, come, and durative (unlimitive), denoting actions of unlimited duration: walk, read, write, shine. With durative verbs the aspect opposition may be neutralized.
So temporal relations in Modem English are expressed by three categories:
1) tense (present - past),
2) prospect (future - non-future),
3) order (perfect - non-perfect)/
The central category, tense, is proper to finite forms only. Categories denoting time relatively, embrace both unites and verbals.
The character of an action is expressed by two categories: aspect (common - continuous) and order.