The irregular imperfect verb stems can be found in a dictionary or on an irregular verb list. For example, if you check the verb trinken (to drink), you will find that it is an irregular verb and ...

But there are irregular verbs! The big two you need to know are ‘to go’ - ‘ir’ and ‘to be’ - ‘ser’. Luckily, they’re both the same in the preterite. They become: ‘fui ...