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Aspect: Another function of auxiliary words is to indicate aspect. Most standard English speakers use aspect without really thinking about it, or sometimes even recognizing it. Aspect in English has to do with whether an action is complete, or incomplete,or ongoing.
The be auxiliary is used to indicate on-going action (also called progressive verbs), as well as action that is/was incomplete:
Elvis is baking six pies. (He's doing in now and the action is both continuous and incomplete.)
Elvis was baking pies. (His action was continuous and on-going)
Elvis is always baking pies. (His action is continuous and habitual.)
The have auxiliary indicates action that was completed in the past.
The circus has come to town. (The action is completed, and it still affects the present.)
The game had already started when the storm broke. (The action was completed before another event.)
Note that more than one auxiliary can be used; this will happen when the verb phrase needs to indicate multiple features -- mood, tense, aspect, or voice.
We were being tested on all six chapters.
They might have eaten all the pie.
Here, in the first verb phrase, we have a be-auxiliary were (indicating on-going action) and a second be-auxiliary being (indicating the passive voice).
In the second verb phrase, we have a modal auxiliary might (giving the mood), and a have-auxiliary, have, indicating that the action is completed in the past and still affecting the present.