This article is merely introductory, and deals only with grammatical aspect in English. Fuller treatments, with cross-language discussion and references to the vast and fascinating technical literature, may be found in the articles on Aspect at Wikipedia, at the Grammatical Features website,and at the Council of Applied Linguists & Language Educators website. The last is part of an under-construction series on TAMPA—Tense, Aspect, Mood, Perfection and Aktionsart—all of which are worth reading. I also urge those whose first language is not English to investigate the expression of aspect in their own languages.

Aspect

‘Aspect’, like tense, signifies the use of grammatical forms to express time-related matters. Like tense, aspect in English is usually ‘attached’ to verbs. But aspect and tense express very different things.

• Tense tells When—the location at which an utterance presents an event as occurring in time.
• Aspect tells How—the manner in which an utterance presents an event as occurring over time.

Also like tense, the term aspect is used in different senses. I will use it here only in the sense of “a formal grammatical category” which categorizes utterances. However, the term is often used more loosely to denote specific grammatical forms, such as the ‘Perfect Aspect’ for the HAVE + VERB^P_A·P_PL construction.

It’s important that you keep these meanings distinct. They are related; but there is no consistent relationship between abstract aspects and concrete verb forms, even when they have the same names. The ‘Perfect Aspect’ has nothing to do with perfective aspect. I’ll have more to say about this later.

There are two sorts of aspect: viewpoint aspect and lexical aspect.

Viewpoint aspect

A fundamental aspectual distinction (most languages grammaticalize it in one way or another) has to do with whether we ‘see’ the event which a verb names as a whole (perfective aspect) or as a continuing process (imperfective aspect). In English this distinction is most clearly visible in the contrasting uses of the ‘simple past’ and the ‘past progressive’.

• perfective aspect
  I built a house, the simple past, looks at the entire action of building a house and sees it as “a single unanalysable whole, with beginning, middle, and end rolled into one” (Comrie). The event is presented as taking place all at once, and the whole event occurs during the extent of time we’re looking at—Event Time (ET), is included in Reference Time (RT) … ET ⊆ RT. In effect we’re looking at this event from the ‘outside’.
• imperfective aspect
  I was building a house, the past progressive, looks at only the portion of the action between its beginning and end. We aren’t told about when the action started or when (or if) it will end. The beginning and end lie outside the extent of time we’re looking at, so RT is included in ET … RT ⊆ ET. In effect we’re looking ‘inside’ the event.
  
  Many scholars divide the imperfective into a number of subcategories: ‘habitual’, ‘continuous’ and ‘progressive’, and there are other viewpoint aspects; but that is all beyond the scope of this post.

As mentioned above, these aspects cannot be consistently aligned with specific verb forms and constructions. For instance:

While the English ‘simple past’ form is perfective, the ‘simple present’ form is ordinarily imperfective—‘habitual’ if used with animate subjects (Susan swims on Thursdays) or ‘generic’ if used with inanimate subjects (The sun rises in the East ). The simple present is perfective only in the relatively uncommon instances when it is used as the ‘historical present’, typically in jokes and sports announcing (An Irishman walks into a bar … . He shoots, he scores!).

Lexical aspect

Lexical aspect expresses the ‘shape’ of the event named by the verb and its arguments, how the event unfolds over time. It is called ‘lexical’ because aspect of this sort is determined primarily by the meaning of the verb itself, the real-life shape of the event which the verb names.

The most fundamental distinction is between states and events:

• States   know, want, believe, like, have.
  Know is a stative verb. When you know something you don’t do anything, nothing ‘happens’; you are merely in a ‘state of knowing’. Know has duration in time (in fact, it is presumed to last indefinitely, unless something happens from the outside to end it); and once you know something you keep on knowing it, without additional effort or energy. Know has no internal ‘stages’ or structure: in particular, it has no target or goal, no point at which you can say your knowing is ‘finished’.

Verbs which denote something which is done or happens, which require effort and energy, are eventive or dynamic. There are a number of tests to determine whether a verb is stative or eventive; here are three fairly dependable ones (but see Aspect shift, below).

• Events may be employed with the progressive construction; states ordinarily may not.

    John is buying me a beer.
  ^∗John is liking beer.

• When employed in a main clause modified by a perfective when clause, events are understood to follow what is described in the when clause; states are understood to start before and continue during what is described in the when clause.

    When I met John he liked beer.
    When I met John he bought me a beer.

• Events can serve as the complement in Wh- cleft constructions; statives cannot.

    What John did was buy me a beer.
  ^∗What John did was like beer.

Grammarians recognize four main sorts of eventive verb, categorized by the presence or absence of two features: duration (Dur) in time, and telicity (Tel), an inherent goal or target state. (Note that the goal must be contained in the verb: that you have a goal is irrelevant, the verb itself must imply reaching the goal.)

• Activities  +Dur, −Tel  study, walk, work, sleep
  Study has duration but no particular goal—it is ‘atelic’. You may have a goal in studying, but the verb does not—if you say you studied English this does not entail a change of state or imply that you reached a goal. You may study mathematics your entire life, you may ‘stop’ studying to eat and sleep, but you never ‘finish’ studying.

• Accomplishments  +Dur, +Tel  learn, make, build, explain
  Learn, like study, has duration, but it is also telic: when you learn something your activity culminates in a transition to a target state, some sort of knowledge or mastery. You learn the multiplication table. This sort of verb is called an accomplishment, because the action is completed by reaching its goal.

• Achievements   −Dur, +Tel  realize, notice, lose, reach
  Realize, like learn, is telic, it embodies the same transition to a state of knowledge as learn. But it has no duration, it does not include the activity leading up to the transition: you can neither start nor finish nor stop realizing. Realize is ‘punctual’, denoting the point in time when the transition occurs.

• Semelfactives   −Dur, −Tel  cough, knock, pop, flash
  Cough has neither duration nor telicity; it ‘happens’, but instantaneously, and it involves no change of state.

Note that there are affinities between specific viewpoint and lexical aspects: states and activities, which have no endpoint, are inherently imperfective, while accomplishments and achievements, which do have endpoints, are inherently perfective.

Aspect shift

Although lexical aspect derives in the first instance from the lexical sense of a verb, it is not confined to that sense. In the first place, many verbs have different senses which present different lexical aspects. For instance:

• Run is ordinarily an activity—I ran for an hour this morning. But if you run a marathon or run an experiment you are engaging in an event which can be ‘finished’—an accomplishment.
• Have in the sense of “possess” is a stative verb—I have a black Toyota. But when you have a good time it becomes an activity.

In the second place, aspect is sensitive to syntax; a particular syntactic use of a verb may compel ‘recategorization’ of its aspect. For instance:

• Finish is ordinarily an achievement—it has no duration, there is a single moment in time at which you finish the task at hand. But when finish is used in a progressive construction—Mary’s finishing the report right now—it is recategorized as an accomplishment, because the progressive compels you to look at finish as a process with duration.
• Build is ordinarily an accomplishment: Mike built a house, employing the perfective past form, looks at the completed action, and Mike is building a house, employing the imperfective present progressive construction, looks inside the action at the process leading up to completion. But when we employ build with an unquantified plural object, as in Mike builds houses for a living or Mike built houses in the 1990s, we recategorize build as an activity which continues indefinitely.
• Knock is ordinarily a semelfactive: Knock on wood.. But knock may be combined with an adverbial or a construction which signifies duration: Frank knocked on the door for ten minutes, Somebody’s knocking at my door. When this is done, we have to ‘redefine’ the verb as a string of repeated actions which together constitute an activity.

Participles employed as adjectives attributing a quality to what is modified in effect compel the verb to be recategorized as a stative, and the same is true of the perfect, progressive and passive constructions which employ participles (as is suggested by the stative auxiliaries which head these constructions).