条件状语从句

用法

在句中做条件状语

连接词

if, unless, in case, as long as, on condition that

要点

if

if引导为最为常见的连接词之一，意为“如果”

-例句
If you are one of them, I would say the only way to change things is by doing rather than complaining.
如果你是他们中的一个，我要告诉你改变事情的唯一办法是做而不是抱怨。

as long as/so long as

as long as/so long as 意为“只要”，强调主句所描述的情况和从句所描述的情况同时存在。

-例句
As/So long as we don't lose heart, we'll overcome any difficulty.
只要我们不灰心，我们就能克服一切困难。

on condition that

意为“条件是”，暗示某种特定的条件。
因为笔者不是很理解，这里给出柯林斯词典解释：
When you agree to do something on condition that something else happens, you mean that you will only do it if this other thing also happens.

-例句
You can use my car on condition that you return it by next Monday.
你可以用我的车，只要你下周一之前还回来就行。

unless

意为“除非；如果不”，相当于 if ... not ...
"if + 肯定句" == "not ... unless + 肯定句"
但是如果 "if ... not" 引导一个虚拟条件状语从句，且谓语动词是be，那么不可互换；and 连接两个 "if ... not" 从句时，不可替换。

unless有时可引出一个过去未曾实现的假想法情况，对前面主句进行补充说明（此时必须置于主句之后，且前面常用破折号而不用逗号）。如：
I couldn’t have got to the meeting on time—unless I had caught an earlier train. 我不可能及时赶到会场，除非我赶上了较早一班火车。（事实是“我”没有及时到会）
此句若将unless改为if...not，则成为：
I couldn’t have got there If I hadn’t caught an earlier train. 如果我没赶上较早一班火车，我就不能到达那儿了。（事实是“我”及时到会了）

-例句
She says that she'll have to close the shop unless business improves.
她说如果生意还没有起色的话，就不得不把店关掉。
Unless you've tried it, you can't imagine how pleasant it is.
如果你不试一试的话，你就想象不出哪有多好。
You can enter the hall if you have a ticket.
You can't enter the hall unless you have a ticket.
你要有门票才能进大厅。

in case

意为“如果，万一”

-例句
In case anything important happens, call me up immediately.
假如有重要的事情发生，马上给我打电话。
In case it pours, do not expert me.
如果大雨倾盆，就不要指望我来了。

其他连接词

let's suppose, suppose 和 supposing (that) 多用于口语，且其主句常为疑问句；peovided (that), providing (that) 多用于正式文体。
-例句
Supposing he does not come, shall we go without him?
如果他不来，我们是不是就不带他去？
Providing you promise not to tell anyone else, I'll explain the secret.
我可以把这个秘密讲给你听，前提是你保证不告诉任何人。