这里的“back”属于副词，表示“回来”的意思。
back
英 [bæk] 美 [bæk]
n.
背，背部;背面，反面;后面，后部;（椅子等的）靠背
vt.
使后退;支持;加背书于;下赌注于
vi.
后退;倒退
adj.
后面的;背部的;以前的;拖欠的
adv.
以前;向后地
例句：
The photographers drew back to let us view the body
摄影师往后退开，好让我们查看尸体。
I went back to bed
我回到床上。
The rail company said it expected services to get slowly back to normal
铁路公司表示，其运输服务将慢慢恢复正常。
She handed the knife back
她递还刀子。
They wrote back to me and they told me that I didn't have to do it
他们回信告诉我说不必那么做。
Can I come back to the question of policing once again?
我能再回到维护治安的问题上来吗？
Short skirts are back
短裙又流行起来了。
Her hair was tied back
她的头发扎了起来。
She lay back and stared at the ceiling
她躺下盯着天花板。
Nick looked back over his shoulder and then stopped, frowning
尼克回头看了看，又皱着眉头停了下来。
Meanwhile, back in London, Palace Pictures was collapsing
与此同时，伦敦的皇宫影业公司也正在分崩离析。
He paced back and forth
他来回踱步。
Her son was lying peacefully on his back
她儿子安静地仰面躺着。
She raised her hands to the back of her neck
她举起双手放于颈后。
He opened the back door
他打开了后门。
There was a neatly folded pink sweater on the back of the chair.
椅子的靠背上搭着一件叠得整整齐齐的粉色毛衫。