on an average . on the average . in most cases；usually通常

On an average Don's mother has to ask him three times before he helps wash dishes．通常唐的母亲叫他三遍他才帮助洗盘子。

On the average the machine turns out one bottle every two seconds．这台机器平均每两秒钟生产一只瓶子。

 on an even keel . in a well ordered way or condition； orderly有条不紊地；安定

 When the discussion seemed to be getting out of hand the chairman got things back on an even keel．眼看争论激烈得要难以收场时，会议主席设法使大家平静下来。

on and on . continuously连续不断地

She talked on and on about her son．她不停地谈到她的儿子。

on approval . with the understanding that the thing may be refused（商品）供试用的；包退包换的

The corporation offered to send us samples on approval．公司提出要向我们提供一些试用样品。

Mr．Grey bought his camera on approval．格雷先生买的相机包退包换。

on behalf of . as a representative代表

John accepted the championship award on behalf of the team．约翰代表球队领取冠军奖。

on behalf of . for the good of为了… 的利益

He rendered many valuable services on behalf of justice and freedom．他为正义和自由做了许多有意义的事情。

They worked hard all their lives on behalf of the poor．他们毕生为穷人的利益而辛劳。

on board . on a ship or train在（船、火车）上

A ship was leaving the harbor，and we saw the people on board waving．船缓缓地离开港口，我们看见船上的人在挥手。

Jack wasn't on board the ship when it sailed．启航时，杰克不在船上。

on business . for the purpose of doing business 有事；因公

Are you here on business or for pleasure? 你来这儿是有事还是消遣呢?

The secretary told the visitor that the manager was out on business．秘书告诉来访者说经理出差去了。

on call . ready and available随叫随到的

The doctors in our clinic are on call day and night．无论白天黑夜，我们医务室的医生总是随叫随到。

The money is payable on call．这笔钱可以随时支付。

 on condition that . if；provided that假如；在…条件下

I will lend you the money on condition that you pay it back in one month．假如你在一个月内能还我钱，我就借给你。

You can go there on condition that you don't come back late for dinner．你可以去那儿，条件是回来吃饭不能太晚。

 on credit . promising to pay at some future time赊欠

I bought it on credit．这是我赊帐买的。

Goods in our shop are not sold on credit but have to be paid for in cash．本店货物现金交易，概不赊欠。

on deck . on the open floor of a ship在甲板上

Shall we go up on deck? 我们到甲板上去好吗?

on deck . ready to do something 准备好

Every employee had to be on deck at nine o'clock．每个雇员都必须在九点钟到。

Everyone was on deck for the dress rehearsal．大家都为彩排作好了准备。

on deposit . in bank存款

The children saved their pennies and each month placed them on deposit．孩子们省些零钱每月存入银行。

He has almost ＄400 on deposit in his account．他帐户上大约有四百美元的存款。

on display . being shown publicly陈列着；在展览

Stop crying if you are not going to have yourself on display in shame！你不想丢人现眼的话就不要哭了！

Samples of all major export commodities will be on display at the fair．主要出口商品的样品将全部在交易会上展出。

on duty . doing one's job ；at work上班

Don't talk with him． He's on duty now．不要跟他说话，他正在值班。

Policemen are not allowed to smoke while on duty．警察上班时不许吸烟。

on earth .  in the world在世界上

Honolulu boasts that it has the best weather on earth．檀香山以气候最佳夸称于世。

They consider themselves the wisest men on earth．他们自认为是世上最聪明的人。

on earth . ever究竟

Where on earth did you find that cap? 你究竟在哪儿找到那顶帽子的?

How on earth did you manage to do that? 你究竟是怎样设法做那件事的?

on easy street . rather rich生活优裕；富裕的

After her success in several films，Mary found herself on easy street．成功地拍摄了几部影片之后，玛丽发现自己的生活颇为优裕。

They have been living on easy street since the family business began to prosper．自从家庭生意开始兴隆以来，他们就过着富裕的生活。

on edge . excited or nervous；irritable兴奋；紧张；烦躁不安

The magician kept the children on edge all through his show．魔术师在他的整个表演中使孩子们兴奋不已。

Most people feel on edge before exams．考试前大多数人感到很紧张。

Everyone was on edge to hear the verdict．每个人都迫切地要听到裁决。

on end . continuously连续不断地

The play is so successful that it has run for years on end．这出戏非常成功，连演几年不衰。

There has been no rain for weeks on end．持续几个星期没下雨了。

on end . upright竖着地

We had to stand the table on end to get it through the door．要让桌子从门里进，我们只好把它侧着竖起来。

Please put the barrel on end．请把这只桶竖起来。