虚拟语气的名词标志
1 “necessity”或“suggestion”等名词后面的表语从句或同位语从句中的谓语动词要用虚拟语气。
2 名词word 表“命令”时，其后主语从句、表语从句或同位语从句中，谓语动词需用虚拟语气 (should)+V。
3 wish 作名词时其后主语从句，表语从句或同位语从句中，谓语动词需用虚拟语气(should)+V。
虚拟语气的形容词标志
It is + adj. that 句型中出现形容词如necessary, important, vital, decisive, critical, crucial, urgent, strange, desirable, advisable, disappointing, 或过去分词充当的形容词如suggested, proposed, advised, demanded, requested, required等时，that 句中用should + V 表示虚拟。
虚拟的介词标志
But for相当于if it had not been for。
But for her help, I would have lost my way.
Without, in the absence of 等组成的短语可相当于一个虚拟的条件从句。
Without the light and heat of the sun, what would become of all the animals on earth?
In the absence of water, plants would not grow well.
Under…condition,也可相当于一个虚拟条件从句。
Under more favorable conditions, we would have made more achievements.
错综时间的虚拟
条件状语从句中谓语动词的形式可与主句中谓语动词的形式不“配套”，即错综时间的虚拟。
If he had done as I had suggested then, he wouldn't be regretful now.
If they hadn't found us then, we would still be missing on the sea.
If I hadn't made sufficient preparation for the exam then, I wouldn't be in college now.
虚拟和事实
(前虚拟后事实)标志: but
Eg. I would have gone to the party with you, but I was too busy.
= If I hadn't been too busy, I would have gone to the party with you.
(前事实后虚拟)标志: otherwise
Eg. I was too busy then, otherwise I would have gone to the party with you.
More Information
was (were) +to have done (不定式完成式)表示“原来想做而未做”后面通常but…(陈述语气)。
Eg. I was to have arrived home on time, but my car was held up by a bad traffic jam.
had hoped表示过去未实现的愿望，其从句中谓语动词要用虚拟would+V。
Eg. I had hoped that he would agree to lend me the money.