Conditional

If

第一类条件句

第一类条件句用于将来可能发生的动作。

If从句中的动词是现在时(一般现在时，现在进行时或现在完成时), 不是将来时. 主句 通常用will或其它的情态动词can, might, should.

• If we hurry, we will catch the bus.

• If you haven't got a television, you can't watch it, can you?

• 第一类条件句可用于提出主动帮忙、建议

  • If you need a ticket, I can get you one.

• 用于警告和威胁

  • If you go on like this, you'll make yourself ill.

有时主句和从句都可用一般现在时, 表示一件事总是自动跟随另一件事发生.

• If you heat water, it boils.

在提出请求时， if从句可以用will.

• If you will just wait a moment, I will find someone to help you.

第二类条件句

第二类条件句用于表示想象的或非现实的情况.

If从句用过去式(一般过去时, 过去进行时), 主句通常用would, could, might.

• If I ate cake, I would get fat.
• If Rachel was playing her stereo, it wouldn't be so quiet in there.

If从句通常不用would, 但在提出请求时， 可用would

• If you'd like to come this way, the doctor will see you now.

在第二类条件句中，有时用were替代was

• If I were you, I'd ask a lawyer for some advice.

第三类条件句

第三类条件句用于表示过去发生的与实际不同的情况, 有时表示对别人的批评或指出别人的错误.

• If you had been a bit more careful, you wouldn't have cut yourself.

从句用过去完成时(had been), 主句常用would have, could have和might have.

第三类条件句也可以用于表示对过去情况的后悔和遗憾.

• If I hadn't fallen ill and missed the interview, I might have got the job.

If, when

If表示认为可能发生的情况, when表示认为将要发生的情况. 在不可能的或假想的情况时, 用if不用 when.

If, Unless

if ... not与unless的意思相同

• I can't see if I don't wear glasses.
• I can't see unless I wear glasses.

in case

用于为了避免以后可能出现的麻烦而做某事

• Take a sandwich with you in case you get hungry.

wish

wish ... would

表示希望某事发生或停止

• I wish you would put those shelves up soon.

wish ... 过去时

表示希望现状有变化

• I wish I lived in a big city. It's so boring in the country.

wish ... 过去完成时

表示对过去发生的事情的愿望

• I wish you had told me about the dance. I would have gone.

If only

If only后面也可接would, 过去时. 它比wish更强烈.

• If only I lived in a big city.