An Overview of the Spanish Preterite:

The preterite is used to describe an action completed in the past, e.g.:  Ayer compré una computadora. Unlike the imperfect, which is used to describe situations in the past, the preterite is used to describe relatively succinct events that have already taken place.

How to Form the Preterite

The preterite is formed by adding an ending to the stem of the verb, e.g.: hablar > habl- + é = yo hablé. The particular ending used depends on the subject (yo, tú, etc.) and the type of verb (-ar, -er, or -ir). Let's first consider -ar verbs, followed by -er/-ir verbs. The endings used are indicated in the following tables:

Verbs ending in -ar, e.g.: hablar

-note that for -ar verb, the nosotros form is the same as in the present (e.g.: hablamos means both "we speak" and "we spoke").

• -ar verbs that end in -car, -gar insert "u" and those that end in -zar change "z" to "c" for the yo forms only, e.g.: yo toqué, yo jugué, yo empecé

Verbs ending in -er or -ir, e.g.: perder, vivir

-note that for -ir verb, the nosotros form is the same as in the present (e.g.: vivimos means both "we live" and "we lived").

• when an -ir or -er verb stem ends in a vowel, the -ió ending changes to -yó and the -ieron changes to -yeron in the 3rd person only, e.g.: él leyó el libro y ellos oyeron las noticias
• -ir verbs that have a stem change in the present have a slightly different stem change in the 3rd person only in the preterite (e>i, o>u), e.g.: yo preferí el pescado pero él no lo prefirió; yo dormí y también ellos durmieron mucho.
• -ar and -er verbs that have a stem change in the present DO NOT have a stem change in the preterite, e.g.: yo pensé que era así, pero él no lo pensó

Irregular Preterite Forms

A small number of highly frequent verbs have irregular preterite forms. Irregular preterites have altered stems and NO accent at the end. The most frequent irregular preterites to be familar with are: ser, ir, dar, decir, hacer, querer, tener and poder.

-other verbs following the j pattern of decir are traer, atraer, distraer, conducir, traducir, and producir

-note that these verbs have the ending -eron, rather than -ieron in the 3rd person plural 

• Note that there is never a final accent on irregular preterite forms.