How do we form the preterite tense?

With regular verbs, remove the -ar, -ir or -er ending, and replace it with the following endings:
For -ar verbs (using comprar 'to buy' as an example)é - compréaste - comprasteó - compróamos - compramos (the same as the present)asteis - comprasteisaron - compraron
For -er and -ir verbs (using comer 'to eat' as an example)í - comíiste - comisteío - comíoimos - comimosisteis - comisteisieron - comieron
Note the position of the accents. Some verbs (like ir, venir and elegir) are irregular and follow slightly different patterns, which we'll discuss another day. Don't worry, there aren't too many and you'll get used to the endings when you use them.