Where are his keys?由于主语keys是复数,所以系动词用are,正确。现改为Where is his key?主语变成了单数key,系动词也随之改为单数is,主谓一致,正确。
如果是单数可以 Where are his keys?指两把钥匙或以上 Where is his key? 指一把钥匙
你这两个句语法都有错误应该是where does his keys你把疑问代词去掉还原就是his are keys和his is keys明显你这两个句子都有语法错误