Cambios de raíz (Stem Changers). Rule stem changes occur stem changes occur in all forms except the nosotros & vosotros forms.
Grammar book
PAST TENSE El Pretérito. El Pretérito: is a past tense (“-ed”) talks about what happened Describes a completed past action.
Stem Changing Verbs Español 1. There is a fairly large group of verbs in Spanish that undergo changes in their stem when conjugated in the present tense.