This is a facet of verb use called "aspect", which receives much less
attention than such things as tense and number.
The chief flavors of aspect are "terminate" and "progressive" (there is
also a "point" aspect). As the names suggest, the terminate aspect
depicts the action in question as completed, while the progressive
depicts it as ongoing:
Last Saturday, I worked in the garden. [terminate]
Last Saturday, I was working in the garden [progressive]
Note that the aspect applies at the time indicated by the tense, not the
time the sentence is uttered. The evolution of the progressive form has
tended to make the terminate form the normal one for stating general or
universal truths ("I get up early").
The progressive form used in the example implies a process still under
way; the suggested terminate alternative requires an implication of
futurity rather than attention to the present. Either is "correct", but
they convey subtly different senses.
--
Cordially,
Eric Walker