21. The door won't open.门不能开。　 open an account 开个账户　 leave the door open 让门开着　 declare sth open　 宣布…开始 be open to the public　 对公共开放opening speech 开幕词

20. only　　 仅仅　　 Only he saw the tiger.　 He only saw the tiger.　　 He saw only the tiger.

Only in this way can you do it well.　　　　 not only… but also…　　 too…to…

only too...to = very　　 I am only too glad to meet you.

19. in one/same direction同一方向one-sided 片面的 one-way 单方面one by one 一个接一个

18. I have been here once.(一次) I once lived here.(曾经)　 once again 再来一次

at once 马上, 同时 I can't do two things at once.　　　 once more 再一次 (once again)

once a day 每天一次　　　 once in a while 偶尔

17. have nothing on 没穿…　 go straight on/ahead　 笔直朝前　　 on the table

There is a new film on on channel 8. 上演 ,　

play a joke on sb 开…一个玩笑　　 in time 及时　　　 on time 准时　　

be based on　　　 on business 因公 on holiday 度假　 on the way to 在途中

on foot　 on tiptoe 悄悄地　　 on hire　

on purpose = by design　　 正好相反　　　 on the other hand 另一方面　

on doing 一…就…　　 On getting off the plane, I recognized him.

16. omit 省略　 You may omit the second paragraph from the article.

15. an official letter　 公函　 　　news from an official source　 官方消息

14. offer to do = make an offer to do 主动提出做…　　　 offer sb sth = offer sth to sb　　 提供　

　　 I offered him ￥100 for his bike. 我出100元买他的车。

13. offend sb　 冒犯　　 be offended with sb　 生…的气　　 be offended at sth

12. see sb off　　 turn/switch off　　 take off 取消，脱衣，起飞，离开　　 be well off 富有

pay off 还清　　 be off duty 不上班　 　　off the point 不在要点

May Day is only a week off.　 五一只有一星期了.