條件狀語從句
用法
在句中做條件狀語
連接詞
if, unless, in case, as long as, on condition that
要點
if
if引導為最為常見的連接詞之一，意為“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.
如果你是他們中的一個，我要告訴你改變事情的唯一辦法是做而不是抱怨。
as long as/so long as
as long as/so long as 意為“只要”，強調主句所描述的情況和從句所描述的情況同時存在。
-例句
As/So long as we don't lose heart, we'll overcome any difficulty.
只要我們不灰心，我們就能克服一切困難。
on condition that
意為“條件是”，暗示某種特定的條件。
因為筆者不是很理解，這里給出柯林斯詞典解釋：
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.
你可以用我的車，只要你下周一之前還回來就行。
unless
意為“除非；如果不”，相當於 if ... not ...
"if + 肯定句" == "not ... unless + 肯定句"
但是如果 "if ... not" 引導一個虛擬條件狀語從句，且謂語動詞是be，那么不可互換；and 連接兩個 "if ... not" 從句時，不可替換。
unless有時可引出一個過去未曾實現的假想法情況，對前面主句進行補充說明（此時必須置於主句之后，且前面常用破折號而不用逗號）。如：
I couldn’t have got to the meeting on time—unless I had caught an earlier train. 我不可能及時趕到會場，除非我趕上了較早一班火車。（事實是“我”沒有及時到會）
此句若將unless改為if...not，則成為：
I couldn’t have got there If I hadn’t caught an earlier train. 如果我沒趕上較早一班火車，我就不能到達那兒了。（事實是“我”及時到會了）
-例句
She says that she'll have to close the shop unless business improves.
她說如果生意還沒有起色的話，就不得不把店關掉。
Unless you've tried it, you can't imagine how pleasant it is.
如果你不試一試的話，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.
你要有門票才能進大廳。
in case
意為“如果，萬一”
-例句
In case anything important happens, call me up immediately.
假如有重要的事情發生，馬上給我打電話。
In case it pours, do not expert me.
如果大雨傾盆，就不要指望我來了。
其他連接詞
let's suppose, suppose 和 supposing (that) 多用於口語，且其主句常為疑問句；peovided (that), providing (that) 多用於正式文體。
-例句
Supposing he does not come, shall we go without him?
如果他不來，我們是不是就不帶他去？
Providing you promise not to tell anyone else, I'll explain the secret.
我可以把這個秘密講給你聽，前提是你保證不告訴任何人。
