When you read that this or that tense "denotes a temporary action" or "denotes a sudden action" or "denotes duration" or "denotes a habitual activity" or any other of a number of impressionistic descriptions, you are reading poetry, in effect. These are only the suggestions of educators who are attempting to give the "flavor" of these tenses to those who, not being native speakers, have no idea what sort of information is conveyed by the use of one tense or another. These efforts are rather like those of a sighted person trying to describe a rainbow to a blind man.
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