[副词]
(1)以前，追溯到；后退，在后面：Please move back:you're in the way. When I
   look back on my days in the army...
(2)回家；在原来的地方：When did you get back? Put the books back on the shelf.
[名词]后面;后背： There is a garden at the back of the house. I've hurt my back.
[形容词]后面的: Sit in the back seat and I'll sit in the front.
[规则动词： back,backed,backed]    
(1)向后移动：Back the car into the garage.
(2)支持:Which team are you going to back?

语法结构
1  动词 | + back
   I looked back and saw someone following me.
   Stand back,and let the doctor through.
   I often think back to my school days.
   表示相反方向的运动，即在身后或追溯到过去.
  
   Are the neighbours back?
   We leave on Thursday and get back on Sunday night.
   He flew there and back in the same day.
   表示返回的运动，即回家、回到原来的地方或是回到其他起点。
  
2  动词 | + 名词短语 | + back
   动词 | + back  | + 名词短语
   What government has ever cut back spending on defence?
   I'll run the tape back,and we'll listen to it again.
   表示反转或翻转东西。
  
   I'm busy right now.Can you phone me back?
   Mrs Jeffries brought back that pan she borrowed.
   Don't lend money to George:he never pays it back.
   表示回复或归还(东西)。

3  动词 | + back | + 介词  |   + 名词短语
   The government promised to raise pensions but it went back on its word.
   The ruins date back to the tenth century.
   表示事情回到原来的状态或倒转
  
搭配
属于语法结构1的短语动词有:
(表示相反方向的运动):fall back,hang back,lean back,lie back,stand back, step
 back,turn back,look back。
(表示返回的运动):arrive back,be back,bounce back,come back,drive back,fly back,
get back,fight back.

Don't hang back:go and ask Fiona for a dance.
   The market experienced sharp falls last week,but it has bounced back in the last few days.
  
属于语法结构2的短语动词有:
(表示相反方向的运动):cut back,keep back,set back.
(表示返回的运动): answer back,call/phone back,bring back,get back,give back,
hand back,pay back,play back,put back,take back,win back.

The spending cuts have set back research several years.
   I take back what I said about Jim:he's proved me completely wrong.
   Did you get your money back?
  
属于语法结构3的短语动词有：cut back on ,date back to , go back on.

My doctortold me I had to cut back on sugar and fat.

还有由动词back构成的短语动词:
从之前说过的某事back down,表示改变了主意.(The government refuse to back down.)         
back into/out of 停车场,表示将车倒进或倒出停车场
back out 你原来同意要做的事，表示你不再同意做它(Hania offered to design my web
page,but she's had to back out.)
back some one up,表示支持某人。 (I'm sure Anita would back me up if she was here.)

固定短语
at the back/in the back = in the rear part of something 在...后面
    There is an index at the back of the book.
    You sit in the back with the kids.

back and forth = going and returning 来来回回
    With mum in hospital,I've been back and forth all week.

back to front =  with the back part at the front 前后颠倒
    You've got your sweater on back to front.

behind my/his etc.back = without my/his etc. knowing 趁我/他等不知道
    When I was on holiday,the architect changed the plans behind my back.