Saber is irregular in the Imperfect Indicative tense.
✻ The Imperfect tense is used when talking about continuous or ongoing action in the past. Something that used to happen or, would (always) happen.
eu ➜ Eu sabia espanhol. (I used to know Spanish.)
você, ele, ela ➜ Você sabia a resposta mas não falou. (You knew the answer but didn't say.)
nós ➜ Nós sabíamos que começaria tarde. (We knew it would start late.)
vocês, eles, elas ➜ Eles sabiam como se comportar. (They knew how to behave.)