Verbs with more than one irregularity
For some verbs there are hardly any learning tools because they have several irregularities; therefore you have to learn them by heart. In addition to an irregular 1st person singular, there are other changes in the stem, such as diphthongation, vowel or stress changes.
The verbs decir, seguir and elegir are irregular in 1st person singular and change their stem vowel from -e– to -i– in all persons except 1st and 2nd person plural.
With the verbs tener and venir the stem is changed besides the 1st person singular. The stem vowel — becomes -ie– in the 2nd and 3rd person singular and in the 3rd person plural.
The verb oler belongs to the group of verbs whose stem vowel -o– becomes -ue-. Before the Diphtong –ue– there is a h-. With the verb errar this is similar. It belongs to the group of verbs whose stem vowel -e– becomes -ie-. With errar the initial i- is written as y-.
With the verbs ir, ser and haber the whole stem is exchanged. Except the plural in ser. Oír contains a -y– in the stem, which only affects the spelling. The pronunciation remains unchanged.