Explanation not a great match

Question

This is the sentence and hint for it talking about ongoing action in the past:Mi abuelo nunca ________ con la ventana abierta.My grandfather never slept with the window open.(HINT: Refers to an ongoing action in the past.)
When you click on 'discuss this' the explanation talks about past in general vs. specific time in the past, not much of a help to understand the ongoing action bit. For me it sounds a lot like repeated action/habit anyway.
Cheers, ALEX

Accepted Answer

Hola Alex
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.
Saludos
Inma