Formas del pretérito

The preterite is the most difficult tense with the most irregularities. There are two principal ways to form the preterite. Here they are called the accented forms and the unaccented forms.

Accented forms

The first, regular system has accented endings in the first and third-person singular forms:
hablécomí viví
hablastecomiste viviste
hablócom viv
hablamoscomimos vivimos
hablasteiscomisteis vivisteis
hablaroncomieron vivieron

Stem-changing -er and -ir verbs.

Verbs that end in -er and -ir that also have a stem change in the present (such as ei as in pido, eie as in siento and ou as in duermo) have a different stem change in the preterite. In these verbs there are only two stem changes, ei and ou occurring in the third person singular and plural forms.

Spelling conventions in the preterite.

Spelling conventions dictate that a verb with a z before an a or an o changes the z to a c before an e or an i. Likewise, a c before an a or an o changes to qu before an e, and a g before an a or an o changes to a gu before an e or an i..
comencépagué toqué
comenzastepagaste tocaste
comenzópagó tocó
comenzamospagamos tocamos
comenzasteispagasteis tocasteis
comenzaronpagaron tocaron

Unaccented forms

The second major system of preterite forms uses irregular stems with unaccented endings. Note that the endings are the same regardless of the type of verb:
estuvepuse vine
estuvistepusiste viniste
estuvopuso vino
estuvimospusimos vinimos
estuvisteispusisteis vinisteis
estuvieronpusieron vinieron