Preterite of Stem-changing Verbs
Remember that, in the preterite tense, -AR and –ER verbs do not stem-change!
However, -IR verbs that stem-change in the present tense, will also stem-change in the preterite.
The change only occurs in the 3rd person singular and plural.
EX:
Servir – to serve
serví
servimos
serviste
servisteis
sirvió
sirvieron
Dormir – to sleep
dormí
dormimos
dormiste
dormisteis
durmió
durmieron
The stem-change consists of either e – i or o – u.