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Irregular -ir verbs
ele / ela decide
eles / elas decidem
This verb is regular because it retains the original decid- root for all the pronouns and has the regular -o, -e, -imos, -em ending for the –ir verbs.
However, there are many verbs that end in –ir and are irregular only because of a slight change in spelling. This change can occur in the root or in the ending.
Some examples of irregular –ir verbs that change the spelling in the root are
sentir – to feel (eu sinto, você sente, ele/ela sente, nós sentimos, vocês sentem, eles/elas sentem)
preferir – to prefer (eu prefiro, você prefere, ele/ela prefere, nós preferimos, vocês preferem, eles/elas preferem)
Some other verbs in this category are subir – to go up, dormir – to sleep, repetir – to repeat and medir – to measure.
Some examples of irregular –ir verbs that are irregular because of the ending are sorrir – to smile, (eu sorrio, você sorri, ele/ela sorri, nós sorrimos, vocês sorriem, eles/elas sorriem)
Other verbs that are also in this category are rir – laugh, incluir – to include, possuir – to posses, and produzir – to produce.
Mostly these small changes occur in the present and sometimes imperative tenses only.
Remember some –ir verbs have a completely irregular conjugation, such as the verb ir – to go (eu vou, você vai, ele/ela vai, nós vamos, vocês vão, eles/elas vão).
You can always consult http://www.priberam.pt/dlpo/Conjugar.aspx?pal=subir
This is a great free online dictionary that also shows the conjugations of verbs. Use it whenever you have a doubt, it’s a good way to practice!