An ELT Glossary : Aspect

Aspect is a feature of the verb system which indicates how an event is viewed - eg as temporary or permanent; as completed or ongoing; etc. It is expressed in different ways in different languages - for example in Slavic languages, perfect aspect may be expressed by prefixes, by changes to the root verb, by stress pattern or in other ways. In English however, it is expressed by the use of primary auxiliary verbs (do, be and have).  

English possesses three types of aspect :

a) simple aspect - which sees an event as "whole". When combined with present tense this means the event is seen as  having no fixed completion point (even if in reality the action may finish sooner or later), and when combined with past tense as complete/finished. Simple aspect uses an auxiliary verb only in cases where an operator is needed for eg negation, SV inversion or emphatic or contrastive stress. Examples : John always exaggerates;  I do like those shoes; Did you see Maria yesterday?

b) progressive (also called continuous) aspect sees the event as on-going but temporary - ie already started but with a fixed, predictable or already occurred completion point. It is formed using the primary auxiliary Be (in the relevant tense - present or past) plus the present participle. Examples : I'm expecting a letter from Jane; I was feeling ill yesterday.

c) perfect aspect expresses anterior time - ie something which happened before the reference point, which is assumed to be now if unstated and if present tense is used, or which will otherwise be explicitly stated. It is formed using the primary auxiliary have plus the past participle.of the main verb :  Examples : David has crashed the car (ie at some unspecified time before now); By ten o'clock this morning, I had cleaned the whole house. (Reference point = at ten o'clock this morning)

Pertfect and progressive aspect can be combined, incorporating the meanings of both by using the prinmary auxiliary Be (for progressive aspect) in the perfect form before the present participle.  Examples:  I've been expecting Anne to call for three days (an ongoing event with a predictable termination anterior to the present moment); By seven o'clock, I'd been sitting at the computer for ten hours solid. (An ongoing event, anterior to a stated reference point, with a predictable termination.)

See also : Auxiliary verbs; Stative and Dynamic Verbs