"Yes, he knew."
Translation:Sí, él supo.
supo: it's the perfect past. It's used when you're refering to the specific time he came to know. Like: "ella lo besó, y él supo que lo amaba" (he learned that she loved him during the kiss).
sabía: it's the imperfect past, which means you can't know when he came to know, just that he did: "Él sabía la verdad" (he knew the truth, but you're not saying when he acquired that information).
The red ones are irregular (for varying levels of irregularity): http://www.linguasorb.com/learnspanish/most-common-verbs/