The following group of -ir verbs undergo the same spelling change in the preterite. The o in the stem, that is, the front part of the infinitive which precedes the ending –ir, changes into u in the third person singular and the third person plural of the preterite. In all the other persons, the stem remains unchanged.
Pay careful attention to their conjugations. Notice the parts that change in the stem (in red), the parts that stay regular in the stem (in blue), and the endings common to –ir verbs in the preterite (in black).
Also, notice the stressed vowel “í” in the first person singular, and the stressed vowel “ó” in third person singular.
Spelling change: o —> u
Pay careful attention to their conjugations. Notice the parts that change in the stem (in red), the parts that stay regular in the stem (in blue), and the endings common to –ir verbs in the preterite (in black).
Also, notice the stressed vowel “í” in the first person singular, and the stressed vowel “ó” in third person singular.
Spelling change: o —> u