Servir is an example of a e → ie stem-changing verb e → i stem-changing verb o → ue stem-changing verb u→ ue stem-changing verb

e → i stem-changing verb

Explanation:

In spanish grammar 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 servir (to serve):

The ending is ir

The stem is serv

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 sense, 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

Now for the verb servir (to serve), we are talking about the third case (From e to i). Let's prove it:

1st person singular Yo: sirvo

2nd person singular (informal) Tú: sirves

2nd person singular (formal) Usted: sirve

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

1st person plural Nosotros: servimos

2nd person plural Ustedes (In latinAmerica): sirven

2nd person plural Vosotros (In Spain): servís    

3rd person plural Ellos/Ellas: sirven

Note that in a stem‐changing verb all forms of the verb will undergo a stem change except for nosotros/nosotras (we) and vosotros/vosotras (you).