Which of the following verbs is not a stem-changing verb?

decir

pedir

repetir

vivir

Firstly, is important to note that a verb in infinitive has two parts:

-The ending: ar, er, ir

-The stem, which is everything else (except the ending, of course)

For example, the Spanish verb decir (to say):

The ending is ir

The stem is dec

Now, some Spanish verbs change their stem in a predictable way when they are conjugated. These are called stem-changers  or stem-changing verbs.

In this type of verbs, all forms of the verb will undergo a stem change except for nosotros/nosotras (we) and vosotros/vosotras (you)

There are three basic different ways in which the stem of a verb can change in the present tense:

-From e to ie

-From o to ue

-From e to i

According to the explanation above, the only verb of the list that is not a stem-changer is vivir (to live), because this verb does not follow the rules.

The ending of the verb is ir and the stem is viv.

Let's prove it  by conjugating this verb in present with the diferent personal pronouns. Note the stem viv keeps the same:

1st person singular Yo: vivo

2nd person singular (informal) Tú: vives

2nd person singular (formal) Usted: vive

3rd person singular El/Ella/eso/esa: vive

1st person plural Nosotros: vivimos

2nd person plural Ustedes (In latinAmerica): viven

2nd person plural Vosotros (In Spain): vivís    

3rd person plural Ellos/Ellas: viven