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All verbs go into two categories: regular and irregular. There are many different types of irregular verbs, and the first type is Stem-changing.
Acertar- to be on target, guess right
Apretar- to be tight, squeeze
ascender- to go up, promote
atravesar- to cross
cerrar- to close
comenzar- to begin
confesar- to confess
defender- defend
descender- to go down
despertar(se) - to wake up
empezar- to begin
encender- to light
encerrar- to lock in, contain
entender- to understand
gobernar- to govern
helar- to freeze
merendar- to have a snack, have a picnic
nevar- to snow
perder- to lose
quebrar- to break
recomendar- to recommend
sentar(se)- to sit down
acordarse- to remember
acostarse- to put to bed
almorzar- eat lunch
conmover- to move (emotionally)
contar- to count, tell
costar- to cost
demostrar- to show
devolver- to return, give back
doler- to hurt
encontrar- to find
envolver- to wrap up
jugar- to play
llover- to rain
mostrar- to show, display
oler (o-hue)- to smell
probar(se)- to try, taste
recordar- to remember
resolver- to solve
soler- to be accustomed to
tronar- to thunder
volar- to fly