Question

How do I know when to use preterite or imperfect is Spanish

Answer 1

Preterite indicates a single event ( something that you dosen't do so often )
the imperfect indicates something that you do habitual 

Answer 2

Pretérite is something at a específic time such as "la película duro 1 hora" but the imperfect is something not specific so like "nosotros íbamos a ver la película".