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 (example sentences) (full conjugation)

   
yo
   
mentí
   
(I told a lie)


   
tú
   
mentiste
   
(you told a lie)


   
Ud./él/ella
   
mintió
   
(he told a lie)


   
nosotros as
   
mentimos
   
(we told a lie)


   
vosotros as
   
mentisteis
   
(you guys told a lie)


   
Uds./ellos/ ellas
   
mintió
   
(you all/they told a lie)

pedir (example sentences) (full conjugation)

    
yo
    
pedí
    
(I requested)


    
tú
    
pediste
    
(you requested)


    
Ud./él/ella
    
pidió
    
(you/he/she requested)


    
nosotros as
    
pedimos
    
(we requested)


    
vosotros as
    
pedisteis
    
(you guys requested)


    
Uds./ellos/ ellas
    
pidieron
    
(they requested)

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.