1. Stem-changing verbs come in several varieties
There are
e-ie (perder, pensar, empezar, preferir, etc.)
u-ue (jugar)
o-ue (mostrar, volver, dormir, etc.)
e-i (pedir, servir, repetir)

2. Look! These don’t stem change in the preterit!
But remember – until NOW, stem-changing has only applied to the present tense!
Look! These don’t stem change in the preterit!
Present tense: Preterit tense
perder
pierdo perdemos perdí perdimos
pierdes perdís perdiste perdisteis
pierde pierden perdió perdieron
volver
vuelvo volvemos volví volvimos
vuelves volveis volviste volvisteis
vuelve vuelven volvió volvieron

3. There is a special class of verbs that DOES stem change in the preterit.
These are -ir stem changing verbs.

4. There are 3 types of –ir stem-changers
Type 1: e > i
pedir: to ask for
servir: to serve
competir: to compete
repetir: to repeat
seguir: to follow

5. Type 1 (e>i)
Type 1 stem-changes from e>i in the present tense. In the preterit it still changes, but only in the 3rd person
Compare:
Present tense: (boot) Preterit tense: (flat)
sirvo servimos serví servimos
sirves servis serviste servisteis
sirve sirven sirvió sirvieron

6. Type 2: e>ie preferir: to prefer sugerir: to suggest
requerir: to require

7. Type 2: e>ie
Type 2 stem-changes from e>ie in the present tense. In the preterit tense it only changes in the 3rd person, but it changes from e>i.
Compare:
Present tense (boot) Preterit tense (flat)
prefiero preferimos preferí preferimos
prefieres preferís preferiste preferisteis
prefiere prefieren prefirió prefirieron

8. Type 3: o>ue
dormir – to sleep
morir – to die

9. Type 3: o>ue
Type 3 stem-changes from o>ue in the present tense. In the preterit tense it only changes in the 3rd person, but it changes from o>u.
Compare:
Present tense: (boot) Preterit tense (flat)
duermo dormimos dormí dormimos
duermes dormís dormiste dormisteis
duerme duermen durmió durmieron

10. Food for thought
All verbs that stem change in the preterit tense are –ir verbs
BUT
Not all –ir verbs stem change!!!!
Ex: vivir, escribir, abrir, descubrir, etc. do not stem change. They are regular.

11. -ir stem changing verbs
No you try it: Competir (e>-i; e>i)
competir
competí
competiste
compitió
competimos
competisteis
compitieron

12. -ir stem changing verbs
Now you try it: Repetir (e-i; e-i)
repetí
repetiste
repitió
repetimos
repetisteis
repitieron

13. -ir stem changing verbs
Now you try it: Morir (o-ue; o-u)
morí
moriste
murió
morimos
moristeis
murieron