whats the difference between stem changing in the present tense and the preterite

Answer 1

I'll send this to our Spanish expert, SraJMcGin.

Answer 2

-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

Answer 3

Note that stem-changing verbs in the Present Tense can be -ar, -er, or -ir but in the Preterit, only -ir verbs. There ARE, however, irregular verbs in the Preterit, but that's not the same as "stem-changing" verbs.

I'll flag this and come back later to see if you have additional questions.

Sra