Stem-changing verbs in the preterit

Stem-changing verbs in the preterit

*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"?

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.

ok thanks for explaining