Stem Changing Verbs e:i
The main form of the verb is called the infinitive. The infinitive consists of two parts: the ending and the stem. The ending is the last two letters. There are only three different endings: -ar, -er, and -ir. The stem is everything else, except the ending.
hablar: ending = ar, stem = habl
comer: ending = er, stem = com
vivir: ending = ir, stem =viv
Some spanish verbs are called stem-changers because when they are conjugated, the stem changes in a predictable way. In one group of stem-changing verbs, the letter e in the stem changes to i in all but the nosotros and vosotros forms. This particular type of stem-changing verb is found only in -ir verbs.
él, ella, Ud. pide
ellos, ellas, Uds. piden
For a more complete discussion of verbs that change e:i click here.
Here are some common e:i stem-changing verbs.
to ask for
to scold, quarrel
to color, dye, stain