Decide if the following sentence is grammatically CORRECT or INCORRECT. Weiss er die Oma?

The sentence is incorrect because wissen is used for facts or information, not for knowing a person. To ask whether he knows Grandma, use kennen: “Kennt er die Oma?” (In standard German, the question word would also be written “Weiß” with ß, though “Weiss” is used in Switzerland.)

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