Stem-changing verbs
A stem-changing verb is one that changes its vowel to a dipthong when the stress falls upon that vowel. These verbs are usually distinguished in the dictionary by having their vowel change placed in brackets after the verb:
poder (ue) puedo, puedes puede, podemos, podéis, pueden.
volver (ue) vuelvo, vuelves, vuelve, volvemos, volvéis, vuelven
querer (ie) quiero, quieres, quiere, queremos, queréis, quieren
pensar (ie) pienso, piensas, piensa, pensamos, pensáis, piensan
Unfortunately, these verbs are irregular and they just have to be learned by heart. However, fortunately, they are regularly irregular and follow an easily recognizable pattern, as can be seen from the following paradigm:
poder
1+2+3=6
4 & 5 volver
1+2+3=6
4 & 5 mover
1+2+3=6
4 & 5 1 puedo vuelvo muevo 2 puedes vuelves mueves 3 puede vuelve mueve 4 podemos volvemos movemos 5 podéis volvéis movéis 6 pueden vuelven mueven The easiest way to remember where the changes occur is to number the verb forms from 1-6. 1+2+3=6 -- these forms all change (because the stress falls on the changeable vowel). However, forms 4 & 5 do not fit the formula and therefore don't change. This is also true for the vowel change e > ie:
pensar
1+2+3=6
4 & 5 querer
1+2+3=6
4 & 5 preferir
1+2+3=6
4 & 5 1 pienso quiero prefiero 2 piensas quieres prefieres 3 piensa quiere prefiere 4 pensamos queremos preferimos 5 pensáis queréis preferís 6 piensan quieren prefieren Unfortunately, the first person singular form of many Spanish verbs is irregular. This means that while the formulae outlined above usually hold good, there are some exceptions. Here are two:
tener
venir
4 & 5 1 tengo* vengo* 2 tienes vienes 3 tiene viene 4 tenemos venimos 5 tenéis venís 6 tienen vienen * = irregular form. Sorry! These verbs have to be memorized. There is no other way! They will appear regularly on tests!