The German language has two very different verbs to express that someone knows (i.e. has knowledge of or is familiar with) something:

– The word “kennen” is used primarily for people, things and places. This word expresses familiarity with something that’s been experienced or that the speaker has had contact with. (For grammar nerds: “kennen” is always followed by an accusative object):

“Ich kenne ihn.” – “I know him.”
“Ich kenne Paris.” – “I know/am familiar with Paris.”
“Ich kenne den Plan.” – “I know/am familiar with the plan.”
“Ich kenne Photoshop nicht so gut.” – “I don’t know Photoshop so well.”

– The word “wissen” is used for abstract information or factual knowledge (and is usually followed by a longer phrase):

“Ich weiß wie es funktioniert.” – “I know how it works.”
“Du weißt nicht was die Hauptstadt von Australian ist.” – “You don’t know what the capital of Australia is.”
“Wir wissen es nicht.” – “We don’t know.”

In sum: “Ich weiß dass du ihn kennst.” – “I know that you know him.”