wissen or kennen?
a1English uses 'to know' for both; German splits them: wissen is for facts and information (often followed by a clause), kennen is for familiarity with people, places and things (always with a direct object).
wissen
Facts, information, answers. Usually followed by a clause with dass, ob, wo, wann …
Ich weiß, dass er heute kommt.I know that he's coming today.
kennen
Being familiar with a person, place or thing. Always with a direct object, never with a clause.
Ich kenne seinen Bruder.I know his brother.
The mistake
Ich weiß ihn.
Ich kenne ihn.
A person is not a piece of information you 'know' - you are familiar with them, so it takes kennen.
Rule of thumb
Rule of thumb: a clause or question after it means wissen. A bare object after it means kennen.
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