Learn Russian: Сначала мы долго слушали дождь, а потом засыпали. - At first we listened to the rain for a long time, and then we fell asleep.

This ties right into our lesson on repeated sequences. Even though the English translation says "we fell asleep", the imperfective засыпали tells us this entire sequence was a habit. It means "First we would listen..., and then we would fall asleep" on multiple occasions. If this sequence had only happened once, the perfective уснули or заснули would have been used instead.

Does the word долго also force the verbs to be imperfective?

Yes, for слушали. The word долго (for a long time) focuses on the ongoing duration of an action, which naturally requires the imperfective aspect. However, засыпали is imperfective simply because the whole "listen, then sleep" sequence was a repeated habit, not because of duration.

I always mix up the aspects of "to fall asleep." What is the perfective equivalent of засыпали?

The perfective past forms you will see most often are заснули or уснули (from the infinitives заснуть or уснуть). Note that the written word засыпали can look tricky: засыпа́ли (stress on the A) is the imperfective "were falling asleep," while засы́пали (stress on the Y) is a different perfective verb meaning "covered with" or "poured." Here, context tells us it is the first one!

Why use а потом instead of и потом for "and then"?