"He had a knife on him."
Translation:Měl u sebe nůž.
I don't get it: Why is it "o sobě" but "u sebe"? May someone explain the difference (I get i wrong every time)?
Different cases, different prepositions and different meaning.
In this sentence, "u sebe" uses genitive.
"O sobě" uses locative and means 'about oneself', eg. "Mluvil o sobě" ='He talked about himself.'