1、在这里back是副词.
be back回来,go back回去.副词back修饰动词be、go.
I will be back soon.我很快就回来.
2、you are back by 9 o'clock.这个句子不正确.第一句才对.
3、此外,back还可以作名词“背部,背后”.
形容词“后面的”,He sat in the back seat of the car.他坐在车子后面的座位上.
动词.使倒退；使后退.She backed her car out of the garage.她从车库倒车出来.