在please look after my clothes when i am away.中,为什么i am后面加副词a

e动词后面的成份是表语,名词,介词,介词短语,形容词和副词都可以做表语.副词不仅可以修饰动词,也可以充当表语的成份.
例如：I am here.我在这里.here就是地点副词,放在be动词后做表语.
The fire is out.火熄灭了.
The TV is on.电视是开着的.
My boss isn't in the office.He is out.我的老板不在办公室,他出去了.
Time is up.到时间了.
The sun was down.太阳下山了.
The war was over.战争结束了.