"They knew each other."
Translation:De kjente hverandre.
The verb "to know" actually covers three different Norwegian verbs, so it can be a bit tricky to decide which one to use.
"Å kjenne" deals with familiarity, and is used when you want to express that you know someone. You will also see it used with a preposition, "å kjenne til", which translates to "to know (of)", and can be used to express awareness of or familiarity (but not proficiency!) with a technique or a book for instance. It can also mean to physically feel something.
"Å vite" deals strictly with factual knowledge when used on its own, and is the right choice whenever you want to express that you know something about something or someone. Used with the prepositions "av" or "om", as in "å vite av/om" something, it usually deals more with awareness or familiarity, meaning that you know of/about something.
"Å kunne" deals with proficiency; knowing a language, a song, a trade, how to play an instrument, etc. It's also used when expressing that you know the way (to) somewhere.