When conjugating verbs, some verbs change not only the ending but also the stem. There are 3 types of stem-change verbs: o--ue, e--ie, e--i
dormir = to sleep (it is an "-ir" verb, but also the "o" in the stem changes to "ue")
querer = to want (it is an "-er" verb, but also the "e" in the stem changes to "ie")
pedir = to ask for (it is an "-ir" verb, but also the "e" in the stem changes to "i")
Now you need to know 2 things when you conjugate these verbs:
1. endings (-ar, -er, -ir)
2. stem changes (o--ue, e--ie, e--i)
Let's try to conjugate verbs than change from e to ie:
comenzar (to begin), divertirse (to enjoy oneself; to have fun), empezar (to begin), entender (to understand), perder (to lose), preferir (to prefer), querer (to want), tener (to have) (tener is also irregular in the "yo" form - yo tengo)
entender = to understand (-er endings and e--ie)
Yo entiendo (I understand) Nosotros entendemos (we understand) (no change "ie" in nosotros)