1. in 1972 president nixon placed ________ on international currency markets, thereby ending the centures-old gold standard

여기서 명사구인 the us dollar은 되는데 부정사구인 to the dollar us는 안되는건가요?

2. at the age of forty-three, john f. kennedy became______of the united states.

여기서 the president는 되는데 부정사구인 to the president 왜 안되는건가요 ? ㅠㅠ

그리고

3. spirits seem ____ as an explanation for the diversity of liveing things to animists

여기서 왜 valid는 되고 a valid는 안되나요 차이점이 뭔가요?ㅠㅠ 부탁드립니다.