如目的所指的人与主句中的主语不同，或原主语再次被提到，则必须使用目的从句：

Ships carry lifeboats so that the crew can escape if the ship sinks．

大船上带有救生艇，万一船下沉船员可以逃生。

This knife has a cork¹ handle so that it will float if it falls overboard．

这把刀的柄是用软木做的，万一从船上掉下去，它可以浮在水面上。

A 目的从句常由so that＋will／would或can／could＋动词原形结构来表示。

这里can／could意为will／would be able to：

They make￡10 notes a different size from￡5 notes so that blind people can（＝will be able to）tell the difference between them．

他们把十英镑的纸币和五英镑的纸币做成不同大小，以便使盲人可以区分开来。

They wrote the notices in several languages so that foreign tourists could （=would be able to）understand them．

他们把通知用几国文字写出来，以便让各国游客都能看得懂。

如句子的主要动词是一般现在时、一般完成时或一般将来时，则都可用can和will。如主要动词是一般过去时，则使用could和would。请看上面和下面这些例句：

I light／am lighting／have lit／will light the fire so that the house will be warm when they return．

我点着／正在点着／已点着／将要点着炉子，以便等他们回来时屋子就暖和了。

I have given／will give him a key so that he can get into the house when－ ever he likes．

我已经给／会给他一把钥匙，好让他能随时进来。

I pinned the note to his pillow so that he would be sure to see it．

我把纸条用针别在他的枕头上了，好让他一定能看得见。

There were telephone points every kilometre so that drivers whose cars had broken down would be able to／could summon² help．

每公里都有求援电话点，以便汽车出了毛病时司机可随时呼唤帮助。

如果带can／could的目的从句中省去that，那么这一目的性可能随之消失。He took my shoes so that I couldn’t leave the house（他把我的鞋拿走了，好让我出不了门）意为He took my shoes to prevent my leaving（他把鞋拿走是为了阻止我离开）。但是He took my shoes，so I couldn’t leave the house（他把鞋拿走了，我出不了门）意为He took my shoes；therefore I wasn’t able to leave（他把我的鞋拿走了，所以我不能够出门）。

B so that／in order that／that＋may／might或shall／should＋不定式结构也可构成目的从句。这种结构仅比上面A中所示的结构更为正规，在意思上是相同的。

注意：so that之后可跟will／can／may／shall或它们的过去式。

in order that或that之后只限跟may／shall或它们的过去式。

that除在戏剧、诗歌及文章中外极少单独使用。

关于时态一致的规定与上文所述相同。下文的句子是很正式的：

We carved their names on the stone so that／in order that future genera－ tions should／might know what they had done．

我们把他们的名字刻在石头上，为的是让后代们知道他们的事迹。

These men risk their lives so that／in order that we may live more safely．

这些人甘冒生命危险，为的是让我们生活更安全。

在一般现在时中may比shall更常见，shall现已很少用。但在一般过去时中might或should都可使用。对上述形式要有所了解，但并不见得需要用，因为在一般情况下，用so that＋ can／could或will／would就足够了。

C 表示否定目的的从句由助动词（will／would或should）变为否定式构成：

He wrote his diary in code³ so that his wife wouldn’t be able to read it．

为了不让妻子看懂，他用密码写日记。

He changed his name so that his new friends wouldn’t／shouldn’t know that he had once been accused of murder．

为了不让新朋友们知道他曾被控犯有谋杀罪，他改了名字。

Criminals usually telephone from public telephone boxes so that the police won’t be able to trace⁴ the call．

罪犯常在公用电话亭打电话，好让警察追查不到电话是从哪儿打来的。

表示否定目的的从句常可用to prevent+名词／代词+动名词或to avoid+动名词来代替：

He dyed⁵ his beard so that we should’t recognize him／to prevent us rec－ ognizing him／to avoid being recognized．

他染了胡子，好让我们认不出／为了阻止／为了避免我们认出他。（被动态动名词）

She always shopped in another village so that she wouldn’t meet her own neighbours／to avoid meeting her own neighbours．

她经常去另外一个村子购买东西，以免碰到她的邻居／为了避免碰到她的邻居。

与表示否定目的的从句相比，人们更喜欢用这些不定式短语。