German has just a handful of really irregular verbs like "sein" and "haben" that don't follow any rules. But there are some verbs that are kind of irregular, but in a regular way!
For example, some verbs have a vowel change when conjugated. We call such verbs "strong" verbs.
You have to memorize the verbs that have vowel changes, but there are some general rules!
First: the vowels "i," "o," and "u" never change!
Second, if "e" or "a" are the second letter in the verb (excluding prefixes), they almost always change.
NOTE: THE ENDING DOES NOT CHANGE!!!
The vowel change takes place only in 2nd and 3rd person singular. All other forms are conjugated like normal.
er / sie / es gibt
sie / Sie geben
The vowel change is always: from "e" to "ie" or "i" OR from "a" to "ä"