虚拟语气的表达
If所引导的条件从句的谓语动词用虚拟语气，主句谓语动词也要用虚拟语气
1. 表示与现在的事实相反：
从句的谓语动词用过去时(be一律用were)，主句用should(第一人称)/would(第二/三人称)/could/might + 动词原形
1.If I were you,I would take an umbrella.
如果我是你，我会带把伞。(事实：我不可能是你)
2.If I knew his telephone number,I would tell you.
如果我知道他的电话号码，我就会告诉你。(事实：不知道)
3.If there were no air or water,there would be no living things on the earth.
如果没有水和空气，地球上就不会有生物。(事实：地球上既有空气也有水)
4.If I had any money with me,I could lend you some.
如果我带钱了，我就会借给你些。(事实：没有带钱)
5.If he studied harder,he might pass the exam.
如果他再努力些，就能通过考试了。(事实：没有努力)
6.she looked at me as if I had been a stranger.
她看我的样子好像我是一个陌生人。(事实：我并非陌生人)
2. 表示与过去的事实相反：
从句的位于动词用had + 过去分词，主句用should(第一人称)/would(第二/三人称)/could/might + have + 动词过去分词
1. If I had got there earlier,I should/could have met her. 如果我早到那儿，我就会见到她。(事实：去晚了)
2.If he had taken my advice,he would not have made such a mistake. 如果他听我的劝告的话，就不会犯这样的错误了。(事实：没有听我的话)
3. 表示与将来的事实相反：
从句：①if+主语+were to do 主句：①主语+should/would/could/might+do
从句：②if+主语+did/were主句：②主语+should/would/could/might+do 从句：③if+主语+should+do主句：③主语+should/would/could/might+do
1.If he should come here tomorrow,I would talk to him.
如果他明天来这儿的话，我就跟他谈谈。(事实：来的可能性很小)
2.If there were a heavy snow next Sunday,we would not go skating.
如果下周日下大雪，我们就不能去滑冰了。(事实：下雪可能性很小)
3.If she were to be here next Monday,I would tell her about the matter.
如果她下周一来这儿的话，我就会告诉她这件事的始末。(事实：来的可能性很小)
当虚拟条件句的谓语动词含有were,should,had时，if可以省略，这时条件从句要用倒装语序，即把were,should,had等词置于句首，这种多用于书面语。eg：
Should he agree to go there,we would send him there.
要是他答应去的话，我们就派他去。
