How do you know when to change the stem on a verb , for example - pedir to pido?

Question

How do I know what verbs to change or when to change them?

Please note that it is mandatory on this forum to use correct spelling, grammar, and capitalization in your posts. - Issabela, OCT 10, 2009

Answer 1

Brute force memorization. That is why they are called "irregular" verbs.

Answer 2

By learning them - it's best to memorize irregular verbs. You can find a list of such verbs here - present tense, or here - the preterit.