一、根据从句动作的持续性来区分
1. “主短从长”型:即主句是一个短暂性的动作,而从句是一个持续性动作,此时三者都可用。如:
Jim hurt his arm while [when, as] he was playing tennis.
吉姆打网球时把手臂扭了。
As [When, While] she was waiting for the train, she became very impatient.
她在等火车时,变得很不耐烦。
注意:as用于引出一个持续性动词表示“在……期间”时,其谓语通常只能是那些含有动作和发展意味的动词,一般不能是那些不用于进行时态的动词,所以下面一句中的while不能换为as:
A:I’m going to the post office. 我要去邮局。
B:While you are there, can you get me some stamps?
当你在邮局时,能帮我买几张邮票吗?
2. “主长从长”型:即主句和从句为两个同时进行的动作或存在的状态,且强调主句动作或状态延续到从句所指的整个时间,此时通常要用while。如:
I always listen to the radio while I’m driving.
我总是一边开车一边听收音机。
He didn’t ask me in; he kept me standing at the door while he read the message.
他没有让我进去,他只顾看那张条子,让我站在门口等着。
但是,若主句和从句所表示的两个同时进行的动作含有“一边……一边”之意时,则习惯上要用as。如:
He swung his arms as he walked.
他走路时摆动着手臂。
I couldn’t remember a story to tell the children, so I made one up as I went along.
我想不出有什么故事可给孩子讲了,只好现编现讲。
3. “主长从短”型:即主句是一个持续性动作,而从句是一个短暂性动作,此时可以用as或when,但不能用while。如:
It was raining hard when [as] we arrived.
我们到达时正下着大雨。
When [As] he came in, I was listening to the radio.
他进来时,我在听收音机。
二、根据主句与从句动作是否同时发生来区分
1. 若主句与从句表示的是两个同时发生的短暂性动作,含有类似汉语“一……就”的意思,英语一般要用as 。如:
The ice cracked as [when] I stepped onto it.
我一踩冰就裂了。
He jumped to his feet as the boss came in.
老板一进来他立刻站了起来。
但是,在hardly [scarcely]…when…句式中,不能将when换成as。如:
Scarcely had we arrived, when it began to rain.
我们刚一到就下起雨来了。
2. 若主句与从句表示的是两个几乎同时发生的短暂性动作,含有类似汉语“刚要……就”“正要……却”的意思,英语一般要用as,且此时通常连用副词just。如:
I caught him just when [as] he was leaving the building.
他正要离开大楼的时候,我把他截住了。
Just as [when] the two men were leaving, a message arrived.
就在这两个人要离开的时候,突然有了消息。
三、根据是否具有伴随变化来区分
若要表示主句动作伴随从句动作同时发展变化,有类似汉语“随着”的意思,英语习惯上要用as,而不用when或while。如:
The room grew colder as the fire burnt down.
随着炉火逐渐减弱,房间越来越冷。
As time goes by my memory seems to get worse.
随着时间的流逝,我的记忆力似乎越来越差。
注:若不是引导从句,而是引出一个短语,则用with,不用as。如:
With winter coming on, it’s time to buy warm clothes.
随着冬天到来,该买暖和衣裳了。
The shadows lengthened with the approach of sunset.
随着太阳下落,影子也逐渐伸长。
四、根据从句动作的规律性来区分
若暗示一种规律性,表示“每当……的时候”,英语一般要用when。如:
It’s cold when it snows. 下雪时天冷。
He smiles when you praise him.
你夸奖他时他总是笑笑。
五、根据主从句动作的先后顺序来区分
若主句与从句所表示的动作不是同时发生,而是有先后顺序时,一般要用when。如:
When he arrives he’ll tell us all about the match.
等他到了,他会好好给我们讲讲那场比赛的情况。
When she had finished she waited as though for a reply.
她讲完之后等了等,仿佛是在等候回答似的。
六、根据是否具有“趁机”意味来区分
1. 若从句所表示的“当……的时候”具有“趁机”的意味,则通常用while。如:
Strike while the iron is hot. 趁热打铁。
I tried to slip the note to him while the teacher wasn’t looking.
我想趁老师不注意时把条子偷偷递给他。
有时也用when。如:
He stole the money when no one was by.
他趁旁边无人时把钱偷走了。
The boy looks at her repeatedly when she’s not looking.
那男孩趁她没看他的时候不断地盯着她看。
七、根据是否用作并列连词来区分
when可用作并列连词,表示“这时”;while也可以用作并列连词,表示“而”“却”;但as则没有类似用法。如:
I was just about to go to bed when I heard a knock on the door.
我正要睡觉,听见有人敲门。
One person may like to spend his vacation at the seashore, while another may prefer the mountains.
一个人会喜欢在海滨度假,而另一个人会喜欢在山里度假。
八、根据所引出的省略句来区分
1. as和when之后均可直接跟一个名词,构成省略句;但是while一般不这样用。如:
As [When] a boy, he lived in Japan. 他小时候在日本。
As [When] still a student, he wrote a novel.
当他还是个学生的时候,他就写了一本小说。
2. when和while之后可接现在分词、介词短语、形容词等构成省略句,但as一般不这样用。如:
When [While] reading, he fell asleep.
他看书时睡着了。
When [While] in trouble, ask her for help.
遇到麻烦的时候你就去找她帮忙。
You must study hard while young, or you will regret when old.
你趁年轻时必须努力学习,不然到老了你会后悔的。