A） 与现在事实相反
从句的谓语用动词过去式或were,主句的谓语用would(should) + 动词原形.
If the weather were fine,we would (should) go out for a walk.
如果天气晴朗,我们就要出去散散步.（实际天气不好.）
If I were you,I would not be so angry.
如果我是你的话,我不会那么生气.（实际上,我就不可能是你.）
B） 与过去事实相反
从句的谓语用had+过去分词；be动词用had been.主句的谓语用would(should) have+ 过去分词.
If you had previewed the text,you wouldn’t have failed to answer the question.
如果你有预习课文的话,你就不至于不会回答这个问题.
She would have come if she hadn’t been so busy.
如果她不是这么忙的话,她早就来了.
C） 与将来事实相反
从句的谓语用动词过去式,should +动词原形,或were to + 动词原形；
主句的谓语用would(should,could,might)+ 动词原形.
If it were Sunday tomorrow,I should go to see the film.
如果明天是星期天的话,我就会去看电影.（实际上,明天不是星期天.）
“were to do”,“should do”和“动词过去式”三种不同形式的比较.
If I were to do it,I would do it in a different way.
要是我来做这件事,我会用不同的方式来做.
@……“were to do”的可能性最小.用于书面语.
If she should come,I would ask her for help.
如果她会来,我就会请求她的帮助.
@……“should do”的可能性介于两者之间.
If you dropped the glass,it would break.
如果你把玻璃杯摔在地上,它会被打碎的.
@……“动词过去式”的可能性最大.