There is really no rule or format for these verbs all you need to do is memorize them and they are easy!
Imperfect vs. Preterite Imperfects main rules Habitual-(used to) A repeated activity(not repeated a specific number of times). An activity theat took place in an indefinite period of time. Ongoing/already in progression in the past(was, were happening) Time Weather Descriptions An interrupted action Simultaneous actions Preterites main rules An action done or completed in a definite period of time. A single occurance An action repeated a specific number of times. Actions that describe a chain of events. Sudden changes in mental states. Interrupting action (imperfect…cuando preterite).
Key Words Signaling the Preterite Ayer- yesterday El ano pasado- last year El sabado por la noche- last Saturday night Cuando-when Una vez- one time
Key Words Signaling the Imperfect Mientras-while Cuando-when Nunca-never