條件狀語從句時態(tài)根據(jù)主句決定。主句是一般將來時態(tài)，祈使句或情態(tài)動詞，從句要用現(xiàn)在時態(tài)；主句如果是過去的某種時態(tài)，則從句要用過去的某種時態(tài)。

條件狀語常用的引導(dǎo)詞是if和unless

1、用if引導(dǎo)：if意為“如果”。

例句：I don’t know if it will rain tomorrow. But if it rains tomorrow, I’ll stay at home.不知道明天是否會下雨，但要是下雨的話，我就呆在家里。

2、用unless引導(dǎo)：unless的意思是“如果不”“除非”。

但除了這兩個以外，條件狀語從句也擁有一些其他的引導(dǎo)詞，這些引導(dǎo)詞出現(xiàn)頻率較低，用法偏復(fù)雜，所以不如if和unless知名度那么高。

1. on condition(that)...在……條件下

eg.I can tell you the truth on condition that you promise to keep a secret.
　　You can eat ice cream on condition (that) you won't eat too much.

2. supposing conj.如果，假如

eg.Supposing it rains,shall we continue the sports meeting?
　 Supposing anything should go wrong,what would you do then?

3. provided conj.假如，除非，以……為條件 provided (that) +從句表示一種假設(shè)條件。

eg. He will sign the contract provided we offer more favorable terms.
　　He won't be against us in the meeting provided that we ask for his advice in advance.

for 若非，要不是

eg.But for the rain,we should have a pleasant journey.
　 But for your help,we should not have finished in time.

5. in case 如果，萬一

eg.In case I forget, please remind me about it.

6. as/so long as 只要

eg.As long as you’re happy,it doesn’t matter what you do.
　 As long as it doesn't rain, we can go.

?

?