Even in the past participle form, most irregular verbs begin with ge-. In many verbs, however, there is a stem change. The participle form of irregular verbs ends most often in -en.
As you can see, there really are not any rules for if and how a vowel in the root will change in this tense, which is why you have to memorize irregular verbs.
Verbs which do not have separable prefixes create the past participle without ge-:
There are a few irregular verbs in the past participle tense that evidence special characteristics. These forms will likewise need to be memorized.
Here are a few examples:
Grammar concepts in German:
regelmäßig: A grammatic form is regular when it is created following an established pattern that can be transferred to other words.
unregelmäßig: Irregular forms are those which need to be memorized, as there is no set rule for how one goes from the base form of the verb to the new form. Irregular forms are those which contain special characteristics that do not follow the set rules.