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 ...

Japanese dictionaries list verbs in this form ... So, in IKIMASU (to go), you change KI to KU, and get IKU. There are two irregular verbs that don’t fit the above patterns.