Irregular Conjugations

Some verbs have vowel changes in the present tense for all forms except first and second person plural.  After dropping the endings (-ar, -er, or -ir), the e of the last syllable changes to ie, and o of the last syllable changes to ue.  Some -ir verbs change the e to i, while verbs ending in -uir change the i to y for all forms except first and second plural.

e to ie	o to ue	e to i	ui to uy
pensar - to think querer - to want, like, love cerrar - to close comenzar - to begin despertar - to awaken empezar - to begin entender - to understand perder - to lose preferir - to prefer sentar - to seat sentir - to regret, feel	contar - to count poder - to be able costar - to cost dormir - to sleep encontrar - to find, meet jugar - to play morir - to die mostrar - to show volar - to fly volver - to return	pedir - to ask (for) repetir - to repeat seguir - to follow servir - to serve vestir - to dress	construir - to build

pensar		contar		pedir		construir
pienso piensas piensa	pensamos pensáis piensan	cuento cuentas cuenta	contamos contáis cuentan	pido pides pide	pedimos pedís piden	construyo construyes contruye	construimos construís construyen

A few other verbs are irregular only in the first person singular form. The rest of the forms follow the regular pattern:

traer	to carry	traigo	I carry
salir	to go out	salgo	I go out
hacer	to do	hago	I do
saber	to know	sé	I know
dar	to give	doy	I give
ver	to see	veo	I see
tener	to have	tengo	I have
poner	to put	pongo	I put
decir	to say	digo	I say
valer	to be worth	valgo	I am worth
caer	to fall	caigo	I fall
conocer	to know	conozco	I know
deducir	to deduce	deduzco	I deduce

Generally, verbs that end in -cer and -cir add z before the first person singular ending.