If a verb is a stem changer in the present tense, it will not stem-change in the preterit unless it is an - ir verb. No - ar or - er verbs will stem-change in the preterit.

The verbs mentir (to lie) and pedir (to request) undergo a stem change from (e) to (i). The congugation of the verbs are shown below.

mentir (conjugation)

pedir (conjugation)

Let's look at a few sentences to understand proper use:

Le pedí que me enviara dinero.
I requested him (her) to send me money.

Nos pediste nuestra documentación.
You requested our documents.

Te pidió que lo acompañaras.
He requested you to acompany him.

Nosotros le pedimos la bicicleta a Luis.
We asked Luis for a bicycle.