Strong and weak German verbs
German verbs fall into two families: weak verbs, which are regular and predictable, and strong verbs, which change their stem vowel in the past and often the participle. Knowing which is which tells you exactly where you can rely on rules and where memorising is unavoidable.
Weak verbs: regular and safe
Weak verbs keep their stem and add predictable endings. machen (to do) turns into Präteritum machte, participle gemacht. spielen gives spielte, gespielt. Once you know the pattern (-te for the past, ge-…-t for the participle), you can conjugate any weak verb you meet, including new ones.
Strong verbs: the vowel changes
Strong verbs shift their stem vowel and take an -en participle. singen (to sing) gives sang, gesungen. fahren (to drive) gives fuhr, gefahren. sprechen gives sprach, gesprochen. The changes fall into rough families, sing, sang, sung being the classic shape, but no rule predicts them reliably.
Many strong verbs also change in the present
A subset of strong verbs changes vowel in the du and er present forms too: fahren gives du fährst, er fährt; geben gives du gibst, er gibt; lesen gives du liest, er liest. This makes the singular present of strong verbs a second thing to watch, separate from their past forms.
The mixed verbs
A small group is "mixed": it changes the vowel like strong verbs but takes weak endings. bringen gives brachte, gebracht; denken gives dachte, gedacht; kennen gives kannte, gekannt. There are only a few of these verbs, but they are common enough to learn as a named set.
Memorise the strong ones by repetition
Weak verbs can be generated by rule, so put your memory to work on the strong and mixed ones instead, since those are the forms no rule can derive. Verboteca separates each irregular form into its own spaced-repetition card, concentrating practice on the vowel changes that rules will never give you.