The stem-changing verbs have vowel changes in the second and third person singular. Even though they're irregular, they are still quite predictable. Once you know a verb is in this group, it's not hard to guess how it's conjugated and to remember it. Here are some of the main verbs, listed according to the nature of the vowel change:
a becomes ä
fahren - to drive, ride on a wheeled vehicle (du fährst, er/sie fährt)
tragen - to carry, to wear (du trägst, er/sie trägt)
e becomes ie
lesen - to read (du liest, er/sie liest)
sehen - to see (du siehst, er/sie sieht)
e becomes i
helfen - to help (du hilfst, er/sie hilft)
essen - to eat (du isst, er/sie isst)