Strong' is a historical terms that refers to the of verbs in which the past tense is marked by a change in the base vowel and the non-progressive pariticple by the suffix '-n' oir -'-en', or no ending ...

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 ...