In the present tense, some verbs have a stem change: a letter in its root changes in some of the grammatical persons. There are two types of stem changes:
Pensar (to think)
Yo pienso
Tú piensas
Él/Ella/Ud. piensa
Nosotros/as pensamos
[Vosotros/as pensáis]
Ellos/Ellas/Uds. piensan
This happens in many verbs; it doesn’t matter if they are -AR, -ER or -IR. Note that the e changes to ie in all persons except Nosotros and Vosotros.
Here are some common “e to ie” verbs:
Cerrar - to close
Divertirse - to have fun
Empezar* - to start/to begin
Entender - to understand
Mentir - to lie
Pensar - to think
Perder - to lose
Preferir - to prefer
Querer - to want/to love
Recomendar - to recommend
Sentarse - to sit down
Sentirse - to feel
* Note: the stem change always happens in the strong syllable: empiezo, entiendo, prefiero.
Soñar (to dream)
Yo sueño
Tú sueñas
Él/Ella/Ud. sueña
Nosotros/as soñamos
[Vosotros/as soñáis]
Ellos/Ellas/Uds. sueñan
Again, the o changes to ue in all persons except Nosotros and Vosotros.
Here are some common “o to ue” verbs:
Acostarse - to go to bed/to lie down
Almorzar - to have lunch
Contar - to tell/to count
Costar - to cost
Devolver - to return (an item)
Dormir - to sleep
Dormirse - to fall asleep
Encontrar - to find
Mover - to move (an item)
Morir - to die
Poder - to be able to
Recordar - to remember
Sonar - to sound
Soñar - to dream
Volar - to fly
Volver - to return
Also, the verb jugar (to play, for sports or games) is conjugated this way: yo juego, tú juegas, etc.
Stem-changing verbs do not change their stem in the past tense:
Pensar (to think)
Yo pensé
Tú pensaste
Él/Ella/Ud. pensó
Nosotros/as pensamos
[Vosotros/as pensasteis]
Ellos/Ellas/Uds. pensaron
Soñar (to dream)
Yo soñé
Tú soñaste
Él/Ella/Ud. soñó
Nosotros/as soñamos
[Vosotros/as soñasteis]
Ellos/Ellas/Uds. soñaron
NOTE: Verbs divertirse, mentir, morir, preferir, querer, sentirse, dormir and poder are irregular in the pretérito perfecto tense.
