Hola Valerie

In our reading and writing exercises there are lots of opportunities to come across the imperfect tense. You can see how the tense is used in the texts and realising the context (e.g when it means "used to..." or when it expresses an "ongoing action" in the past) - Sometimes we test how to form the tense, but as you try the exercise you learn its use at the same time. 

If you go to "library" and go to "writing practise" and then "fill-in-the-blanks exercises" on the right, you'll see lots of gapfills. You can start by trying "El nombre de la Alhambra" and "La ilusión de los Reyes Magos". You can find them here.

 

In terms of exercises to practise "when" to use the imperfect, we do have other gapfills that are more advance and that test the imperfect against the preterite - note that these are normally higher in level because there isn't normally enough space in the text to make it absolutely clear whether you need one tense or another. But you can try with one called "Las tapas" for example.

I hope this helps.

Saludos cordiales y feliz año nuevo.

Inma