How do you know when to use the imperfect tense and the preterite tense?

There are two simple past tenses in Spanish: the imperfect and the preterite tenses.

The imperfect tense is used to describe continuous past actions, something which used to happen or to describe what something or someone was like.

The preterite tense is used to describe a one-off past action, or a continuous action with a specific endpoint.