“Aspect” refers to the way the reported event is perceived. English aspect is marked formally and explicitly on the verb. The verb form BE V-ing signals the progressive aspect, while the form BE-V-en signals the perfect aspect. In English, the same event (e.g., He wrote an article last nignt) may be described in two different ways, highlighting two different perspectives of viewing the event: progressive versus perfective, as exemplified in the following:
Two distinct viewpoints
1. Progressive viewpoints
He was writing a book. (an ongoing, unbounded event)
2. Perfective viewpoint
He has written an article. (completed prior to speech time)
Liu, Meichun. 2015. Tense and aspect in Mandarin Chinese. In The Oxford Handbook of Chinese Linguistics, chapter 21, eds. by William S-Y. Wang and Chaofen Sun, 274-289. Oxford: Oxford University Press.