Stem-changing verbs in the preterit

[Jessica]

*Verbs whose infinitive ends in -gar, like pagar, jugar, and llegar, end in -gué in the yo form of the preterite.

--¿Cuándo llegaste al cine anoche?
--Llegué a las siete y media.

*Verbs whose infinitve ends in -car, like buscar, tocar, and sacar, end in -qué in the yo form of the preterite.

--¿Qué buscaste en la tienda ayer?
--Busqué un suéter rojo.

*Verbs that have a stem change in the present do not have a stem change in the preterite.

Generalmente el supermercado cierra a las diez, pero anoche cerró a las ocho.

I don't get this one. What does it mean by "have a stem change in the present but do not have a stem change in the preterite"?

[Rusty]

It means that the stem changes found in several present tense verbs aren't performed when conjugating into the preterite tense.
There are many stem changing verbs. There are three such changes:
e -> ie (querer - quiere)(tener - tiene)(venir - viene)
e -> i (pedir - pide)(servir - sirve)(repetir - repite)
o -> ue (poder - puede)(volver - vuelve)(dormir - duerme)

Those changes don't occur in the preterite tense. However, there are many irregular preterite tense verbs that involve changing the root form. You'll learn about those, too.

[Jessica]

ok thanks for explaining