"He had a knife on him."

Translation: Měl u sebe nůž.

[Shathu_Entayla]

Why "u sebe nuz mel" is nos correct?

[Ilja...]

I don't get it: Why is it "o sobě" but "u sebe"? May someone explain the difference (I get i wrong every time)?

[endless_sleeper]

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.'