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if he has time, he will go with us.= probably he has time and will go with us.
if he had time, he would go with us.=but in fact he has no time.
1. 行为动词用did 形式
could + 动词原形
had + done
could + have + done
2. should + 动词原形
3. were to + 动词原形
could + 动词原形
1)0if i had received the passport yesterday, i would start today.
2)if he had telephoned me last night, i would see him now.
3)if he had followed the doctor’s advice, he would be all right now.
4)if china had not been liberated, the working people would still be leading a miserable life.
4.should/ could / might/ ought to + have done 表示“过去本应该/可以做而实际上却没做”
needn’t have done 表示“过去没必要作而实际上做了”
如果虚拟语气的条件从句谓语动词中含有were, had, could, should，有时可将if省去，而将条件从句的主语置于were, had, should, could 之后。
had you invited us, we would have come to your party.
were i you, i would do more practice after class.
could she lend us a helping hand, she would do so.
6.wish 后面的宾语从句的谓语动词应使用虚拟语气， 表示“可惜…；….就好了； 悔不该…； 但愿…。”
动词用would do; should do
be 用 would be ; should be
1)i wish i knew the key to the answer.
2)i wish i were ten years younger.
3)i wish that i had gone to the football match last week.
4)i wish that you had been here yesterday.
5)he wishes that we would visit the old school.
7.表示命令或建议动词suggest, insist, propose, desire, demand, request, order, command 后的宾语从句中应使用虚拟 should +动词原形； should 不可用would 来替代; 主句所使用的动词时态不限。
8.suggest 为“建议去做…; 命令…”从句用should + do
为“ 说明； 暗示”, 从句用过去时或过去完成时。
1) the doctor suggested that i should take the medicine three times a day.
2) the doctor suggested that my grandmother had caught a bad cold.
9.insist “坚持要去做…，坚持应该去做”，从句用should + do为“坚持表明，坚持说/解释”， 从句用过去时或过去完成时。
10.虚拟语气也用于表语从句和主语从句中，表示间接的命令、要求、请求、建议、决定等，主句的主语通常是suggestion, proposal, request, order, idea 等。表语从句中的谓语动词是should +动词原型，should 可以省略。
11.在主语从句中，当从句用来表示惊奇、不相信、惋惜等，从句的谓语动词用需拟语气形式。其谓语动词时should + 动词原型， 或should 省略。