Stem-changing verbs arranged according to "classes":

CLASS I  (stem-changing verbs that end in -ar and -er and have a change of o>ue  or e>ie in present tense) [These verbs change ONLY in Present Indicative and Present Subjunctive.]:

recordar

perder

mover

pensar

presente de indicativo                    

CLASS II (stem-changing verbs that end in -ir and have a change of o>ue or e>ie in present tense).  Changes also are made in preterit indicative, present subjunctive, imperfect subjunctive, and the present participle.

morir

dormir

sentir

preferir

CLASS III (stem-changing verbs that end in -ir and have a change of e>i in present tense).  Changes also are made in preterit indicative, present subjunctive, imperfect subjunctive, and the present participle.

pedir

vestir

servir