The –er and –ir verbs like leer, creer, caer, construir, destruir and oír are irregular in the preterite because they require a spelling change in the third person singular and the third person plural. This change consists on replacing the vowel i with a y because an unaccented i between two strong vowels a, e, u changes into a y. This contact of the weak vowel (i) with the strong vowel (o) comes into place when the endings of the preterite (-í, -iste, -o, -imos, -isteis, -ieron) are added to the stem of the infinitive. In the verb leer, the third person singular is not leió but leyó, and the third person plural is not leieron but leyeron. When conjugating these verbs, apart from paying attention to this double spelling change, notice the accents. Below are the conjugations for your study.