Diphthongization
Some verbs with -e- or -o- in their stem are inherently diphthongizing, while others are not: their identities must be learned individually. In a diphthongizing verb, the change turns -e- into -ie- and -o- into -ue- when the syllable in question is stressed, which in effect happens only in the singular persons and third-person plural of the present indicative and present subjunctive, and in the imperative (all other tenses and forms are stressed on their endings, not their stems). Note that the dictionary form always has the vowel, not the diphthong, since, in the infinitive form, the stress is on the ending, not the stem. Exceptionally, the -u- of jugar and the -i- of adquirir also are subject to diphthongization (juega, etc.; adquiere, etc.).
Note also that, when word-initial, *ie- is written ye- (errar > yerro) and *ue- is written hue- (oler > huelo, desosar (des-osar) > deshueso). Also, the -ue- diphthong is logically written -üe- after g: avergonzar > avergüenzo.
The following examples show that all three conjugations (-ar, -er, and -ir verbs) include some diphthongizing verbs (only some tenses and persons are shown, for contrasting purposes):
Read more about this topic: Spanish Irregular Verbs, Stem-vowel Changes