有三种类型的条件句

类型	条件
I	可能满足的条件
II	只是在理论上满足的条件
III	不可能满足的条件 (或者说事情已经发生)

构成

类型	条件从句	主句
I	一般现在时	将来时(或者 情态动词 + 不定式)
II	一般过去时	would + 不定式 *
III	过去完成时	would + have + 过去分词 *

例句

类型		例子
I	肯定	If I study, I will pass the exam.
	否定	If I study, I won't fail the exam. If I don't study, I will fail the exam.
II	肯定	If I studied, I would pass the exam.
	否定	If I studied, I wouldn't fail the exam. If I didn't study , I would fail the exam.
III	肯定	If I had studied, I would have passed the exam.
	否定	If I had studied, I wouldn't have failed the exam. If I hadn't studied, I would have failed the exam.

* 我们能用 could 或 might 去替代 would (should, may 或 must 某些情况下也可以).

I would pass the exam.

I could pass the exam.

I might pass the exam.

I may pass the exam.

I should pass the exam.

I must pass the exam.