Strong stem preterits: A special class of verbs are referred to as strong stem preterits since they have a special stem which is “strong” enough to carry the stress in the yo and él/ella/usted forms, instead of the ending being stressed such as occurs in the forms hablé and habló.
These verbs also have a special set of endings, none of which carries a written accent mark:
-e, -iste, -o, -imos, -isteis, -(i)eron
Ej. andar: anduve, anduviste, anduvo, anduvimos, anduvisteis, anduvieron
Some of the most frequently used verbs in this class are:
andar: anduv- to walk, go poner: pus- to put, place
conducir: conduj- to drive, lead
querer: quis- to want, love estar: estuv- to be haber: hub- to have traducir: traduj- to translate hacer: hic- to do, make traer: traj- to bring poder: pud- to be able venir: vin- to come
* If the preterit stem ends in a -j, the -i of the third person plural form is dropped: dijeron, trajeron.
**The third person singular form of hacer has a spelling change: hice, hiciste, hizo, hicimos, hicisteis, hicieron
Irregular verbs: Some verbs are completely irregular in the preterit:
dar: to give di diste dio dimos disteis dieron
ir: to go fui fuiste fue fuimos fuisteis fueron
ser: to be fui fuiste fue fuimos fuisteis fueron
Note that the preterit forms of ir and ser are identical; context should determine which verb is being used.