-ar and -er verbs that change their stem in the present tense do not change in the preterite. They are conjugated just like other regular preterite verbs, using the regular endings.
cerrar
cerré
cerraste
cerró
cerramos
cerrasteis
cerraron
entender
entendí
entendiste
entendió
entendimos
entendisteis
entendieron
-ir verbs that change their stem in the present tense do change in the preterite, but in a different way. They change e:i and o:u in the third person, singular and plural.
preferir
preferí
preferiste
prefirió
preferimos
preferisteis
prefirieron
dormir
dormí
dormiste
durmió
dormimos
dormisteis
durmieron
