Verbs with vowel change in the stem
Many irregular verbs have changes in the stem vowel. These changes include the formation of a double sound (diphtongization), vowel changes and stress changes.
That the 1st and 2nd person plural have no stem vowel changes is due to the emphasis. Only the stem stressed forms are changed.
Changing the end vowel
In addition to the change of the stem vowel, some verbs of the -ir conjugation replace the -i- of the ending with a -y-. However, this only applies to verbs whose stem ends on an unstressed -u- (e.g. constru-ir).
Change of stress
In a group of verbs that end on – iar and -uar, the emphasis changes from -i- or –u-. In this group there are only verbs of -ar conjugation. The otherwise unstressed vowel -i- or -u- at the end of the verbal stem is emphasized.