if从句遵循什么原则;if从句与主句的时态

英语条件从句的使用

在英语的状语从句中，有一种是条件状语从句，条件句子是讨论在某种因素或假设情景下的声明，完整的条件句一般由If引导从句，然后是结果的主句。

If a certain condition is true, then a particular result happens.

I would travel around the world if I won the lottery.

When water reaches 100 degrees, it boils.

有四种条件从句，每个从句表达的含义不同。注意由于条件的模式不同，造成条件从句的时态不一。每种条件表达的可能性不一样，造成某种情景发生的可能性不同。

第一条件状语从句

零条件句子表示一般性的事实，即一件事发生必定会导致另一件发生。例如：

If you don’t brush your teeth, you get cavities.

When people smoke cigarettes, their health suffers.

在上面第一条件的句子中，首先从句的时态一定是一般现在时，不能用将来时，同时主句也是一般现在时。但主句不能用一般将来时，例如：

If people smoke cigarettes, their health will suffer. （错误）

其次，在第一条件句子中when和if是可以互换的，因为结果总是相同，即可以说：

When people smoke cigarettes, their health suffers.

也可以说：

If people smoke cigarettes, their health suffers.

第二条件状语从句

这种条件状语从句表示后果在将来会发生，虽然不像第一条件100%发生，但是大概率事件。例如：

If you rest, you will feel better.

If you set your mind to a goal, you’ll eventually achieve it.

由此可以看出If从句用的是一般现在时，从句用的是一般将来时，表示在给定条件下，这个事情会发生。从句不能用一般将来时：

If you will rest, you will feel better. （错误）

If you rest, you will feel better. （正确）

第三条件状语从句

第三条件状语从句是表达结果完全不现实或在将来不可能发生。例如：

If I inherited a billion dollars, I would travel to the moon.

If I owned a zoo, I might let people interact with the animals more.

注意在构造第三种条件注意If 之后用的是一般过去时，而主句用过去的情态动词 (例如could, should, would, might) 来表达不现实的结果。

根据这个原则，下列句子各有一处是错误的，你看出来了吗？

If I inherit a billion dollars, I would travel to the moon.

If I owned a zoo, I will let people interact with the animals more.

第四条件状语从句

第四种条件表示现在的结果会截然不同，如果当初某事是另一个结局，有些吃后悔药的感觉。例如：

If you had told me you needed a ride, I would have left earlier.

If I had cleaned the house, I could have gone to the movies.

这两个句子在条件中表达了某种可能，但事实上过去没有这么做，根本就没有发生，因此产生的结果当然不同。虽然条件是可能的，但很遗憾没有发生。

注意当用第四种条件句时，从句If用的是过去完成时 (就是 had + 过去分词)，主句用的结构形式是(would, could, should, 等) + have + 过去分词，表达可能发生的结果。实际上这是虚拟语气的运用，关于虚拟语气详见另一篇头条文章。

条件从句的几个特例

一般来说在第二种条件从句中，符合“主将从现”，即主句是一般将来时，从句用一般现在时，但有一种情况是主句是条件，当主句发生后，从句的条件会变真，即主句动作在前，从句动作在后，那么从句用一般将来时。例如：

If aspirin will ease my headache, I will take a couple tonight.

还有一种were to形式的条件从句，用来表达结果不情愿。例如：

If I were to be sick, I would miss another day of work.

If she were to be late again, she would have to have a conference with the manager.

If the rent were to have been a penny more, they would not have been able to pay it.

条件状语从句其它表达

Unless引导的条件

例如：

Unless I phone you, you can assume the train’s on time. (If I do not phone you /except if I phone you, you can assume the train is on time.)

We’ll have to cancel the show unless we sell more tickets at the last minute. (We’ll have to cancel the show if we do not sell more tickets/except if we sell more tickets at the last minute.)

Should引起的倒装句型

例如

Should you wish to cancel your order, please contact our customer service department on 02317 6658932. (或者 If you should wish to cancel your order …)

Should your child become anxious or nervous about any activity, it is a good idea to inform the team-leader. (或者 If your child should become …)

Had引导的倒装

例如

Had I known you were waiting outside, I would have invited you to come in. (If I had known you were waiting outside …)

Had Margaret realized she would be travelling alone, she would never have agreed to go.

Were to引导的倒装结构

例如

Were we to give up the fight now, it would mean the end of people’s interest in our country. (If we gave up the fight now …)

Were the economy to slow down too quickly, there would be major problems. (If the economy slowed down too quickly …)

某些词组的引导的条件

这样的词组有：

As long as 只要

So long as 只要

On condition that 若

Providing 只要

Supposing 假使

You can play in the living room as long as you don’t make a mess.

So long as a tiger stands still, it is invisible in the jungle.

The bank lent the company 100,000 pounds on condition that they repaid the money within six months.

You can get a senior citizen’s reduction providing you’ve got a rail-card.

They may do whatever they like provided that it is within the law.

Supposing I don’t arrive till after midnight, will the guest-house still be open? (Imagine if I don’t arrive till after midnight …)

Supposing you lost your passport, you’d have to go to the embassy, wouldn’t you?

Supposing he hadn’t recognized us – he might never have spoken to us.

有时or或otherwise也有表达条件与结果的关系, 例如：

You’ve got to start studying, or you’ll fail all those exams. (If you don’t start studying, you will fail the exams.)

We’d better send it express, otherwise it’ll take days. (If we do not send it express, it will take days.)