the 'e' goes to 'ie' if the 'e' is in an accented syllable

calentar - to heat up, to warm up, to turn on sexually
acertar - to choose well
acrecentar - to increase
confesar - to confess
alentar - to encourage
apretar - to press, squeeze, tighten
arrendar - to lend, to lease
aventar - to fan
cimentar - to solidify
concertar - to agree on, to reach
desalentar - to dishearten, to discourage
deshelar - to thaw, to melt, to de-ice
despertar - to wake, to arouse
desterrar - to banish, to exile
encerrar - to shut up, to shut in
escarmentar - to learn one's lesson (I'm gonna teach you a lesson...)
invernar - to winter, to hibernate
mentar - to mention
pensar - to think
quebrar - to break
sentar - to sit, to seat

the 'e' changes to 'ie' if it is in an accented syllable

atender - to attend to
cernir - to sift, to sieve
condescender a - to consent to
contender - to contend, to fight
defender - to defend, to stand up for
desatender - to neglect, to ignore
descender - to go down
distender - to ease, to slacken
encender - to light, to arouse, to inflame
extender - to stretch out, to
perder - to lose
discernir - to distinguish, to discern
concernir - to concern
adquirir - to acquire, to purchase
proferir - to utter, to hurl (insults)