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) in the "Ud./él/ella" and the "Uds./ellos/ellas" forms only. The conjugation of the verbs are shown below.

mentir (conjugation)

pedir (conjugation)

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

Pedí un vaso de agua.
I asked for a glass of water.

Tú pediste zapatos negros.
You asked for black shoes.

Juan pidió un vaso de agua.
John asked for a glass of water.

Nosotros pedimos carros nuevos.
We asked for new cars.

José y Carmen pidieron una soda al mesero.
José and Carmen ordered a Soda from the server.

Other verbs that follow this pattern are:

hervir (to boil)
preferir (to prefer)
sentir (to feel)
convertir (to convert)
divertirse (to enjoy yourself)
seguir (to follow)
medir (to measure)
repetir (to repeat)
impedir (to prevent)
reñir (to argue).