Нужно употребить во в косвенной речи
He doesn't know (Who was the nineteenth president of the United States?). He isn't sure (When did the Civil War end?), He doesn't remember (When did California become a state?), He has forgotten (Where was George Washington born?). He can't remember (How many people signed the Declaration of independence?). He doesn't know (Where was Abraham Lincoln assassinated?). He has forgotten (Why was Washington chosen as the capital?). And he has no idea (What did Alexander Graham Bell invent?).
He doesn't know who the nineteenth president of the United States was. He isn't sure when the Civil War ended. He doesn't remember when California became a state. He has forgotten where George Washington was born. He can't remember how many people the Declaration of Independence signed. He doesn't know where Abraham Lincoln was assassinated. He has forgotten why Washington was chosen as the capital.He has no idea what Alexander Graham Bell invented.