2017年6月英语四级语法知识：条件句

　　虚拟条件句主要有下面两类:

　　表示现在及将来情况(表示纯然假设或实现的可能性不大的情况):

　　谓语主要形式如下(be多用were这个形式):

　　从句 主句

　　过去式

　　would + 动词原形

　　if i were you, i wouldn’t lose heart.

　　how nice it would be if you could stay a bit longer.

　　表示过去情况的虚拟条件句(与事实完全相反的假设情况), 谓语主要形式如下:

　　从句

　　主句

　　had + 过去分词

　　would have + 过去分词

　　she would have come if we had invited her.

　　if i hadn’t taken your advice, i would have made a bad mistake,

　　you wouldn’t have caught cold if you had put on more clothes.

　　有时候, 条件从句表示的动作和主句表示的动作, 发生的时间是不一致的(如一个是过去发生的, 一个是现在发生的). 这时, 动词的形式要根据表示的时间来调整. 这种句子可以称为错综时间条件句.

　　if he had received six more votes, he would be our chairman now.

　　if we hadn’t got everything ready by now, we should be having a terrible time tomorrow.

　　有时假设的情况并不以条件从句表示, 而是通过一个介词短语来表示.

　　without music, the world would be a dull place.

　　we could have done better under more favorable conditions.

　　that would have been considered miraculous in the past.

　　but for the storm, we should have arrived earlier.

　　如果条件句从句中包含有 were, had, should 或could, 有时可把if省略掉, 并把were, had, should或could放在主语前面.

　　had we made adequate preparation, we might have succeeded.

　　should there be a flood, what should we do?

　　were it not for their assistance, we would be in serious difficulty.