the 'u' changes to 'ü' if the conjugation gives the stem an 'a', an 'e', or an 'o'

the 'u' changes to 'ü' if the conjugation gives the stem an 'a', an 'e', or an 'o'

aguar - to water down
amortiguar - to muffle, soften, cushion
apaciguar - to calm down, to soothe (got to love that valium)
atestiguar - to testify to
averiguar - to find out
fraguar - to forge
menguar - to decrease, to diminish
agorar - was not listed in the dictionary
errar - to choose wrong
desosar - was not listed in the dictionary
oler - to smell
poseer - to own, to possess
proveer - to supply, to provide