When should you use the preterite tense and when should you use the imperfect tense in Spanish

Both the preterite and imperfect tenses are past tenses in Spanish but they are used in separate ways.

Imperfect Tense

1. Habitual actions 

2. "ing" e.g I was walking 

3.  what you were doing when interrupted by something else.

Preterite Tense 

1. Actions at a given moment or time

2. past completed action 

3. actions that interrupt another action 

Both tenses can be used in the same sentence e.g I was walking when I heard a sound - Caminaba cuando escuché un sonido.