The following verbs are all irregular in the preterite and are grouped in 3 different categories according to the irregularity they share in the stem (front part of the infinitive which precedes the endings -ar, -er, and -ir).
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“u”-stem verbs: ALL the persons have the vowel “u” in the stem of the preterite.
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“i”-stem verbs: ALL the persons have the vowel “i” in the stem of the preterite.
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“j”-stem verbs: ALL the persons have the consonant “j” in the stem of the preterite.