Stem-vowel Changes
There are two kinds of changes that can affect stem vowels of some Spanish verbs: diphthongization and vowel raising. Both changes affect -e- or -o- in the last (or only) syllable of a verb stem. Diphthongization changes -e- to -ie-, and -o- to -ue-. Vowel raising changes the mid vowels -e- and -o- to the corresponding high vowels: -i- and -u- respectively. Some verbs, in their various forms, can exhibit both kinds of changes (e.g. sentir, siente, sintió; dormir, duerme, durmió).
Read more about this topic: Spanish Irregular Verbs