downstair 只是形容词,这时和downstairs 用法相同.
downstairs 可以做adj之外,还可以做副词和名词.
eg : He is in a downstair(s) room.
Let's go downstairs, shall we?
当做副词用做定语时,放在修饰词之后.
eg : The man downstairs can't stand the man upstairs.