When we say in the lesson that "El Pretérito Imperfecto tells us in general when an action took place, whereas El Pretérito Indefinido tells us specifically when an action took place." When we say "in general" we mean that it's not relevant when the action took place, in contrast with the Indefinido which expresses something that happened at a specific time, day, time frame, seen as a completed action with an end.
We continue to say later on in the lesson that "In the examples from above, the time markers tend to trigger El Pretérito Imperfecto because they generally refer to an ongoing length of time or express frequency with no definite beginning and end."
There is also another lesson about the contrast between these two tenses but without bearing in mind any time markers. This could also help. Have a look here.
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