if条件句
1. 真实条件句：非虚拟（发生可能性大）
主将从现：if it rains tomorrow, we will put off the sports meeting
2.非真实条件句：虚拟（发生可能性不大）
与现在事实相反
if 主语+一般过去时(did/were)，主语+would/could/might/should/ought to+V原形
if i were you, i would not marry him
与过去事实相反
if 主语+had done，主语+would/could/might/should/ought to have +过去分词
If i had been you, i would not have married him.
情态动词+have done：表示与过去事实相反 例如should have done本不应该做但做了
与将来事实相反
if 主语+should/were to +V原形，主语+would/could/might/should/ought to+V原形
if i should be you, i would not marry him
if 主语+should/were to +V原形，主语+will/can/may/should/ought to+V原形
if i should be you, i will not marry him
特殊词引起
1. 命、要、建、定、愿
一个单词的词根引起了虚拟语气，那么它的其他所有形式都会引起虚拟语气，不考虑形式特征，只考虑表达含义。从句中都用should do
表愿望：desire, prefer, wish, hope
表建议：advise, propose, suggest, recommend, motion
表要求请求：ask, beg, insist, maintain, oblige, request, require, urge, petition
表命令：command, decree, dictate, direct, order, stipulate
表决定：decide, determine, resolve
表其他：arrange, deserve, intend, motion, promise, inveitable,
2. 特殊adj: important/ necessary
写作中不要使用从句，用 it is important for sb to do，避免出错
特殊用法
1. it's time......
此句型后用that从句时，应为“主语+did”
it's time that you went to school
2. wish
与现在愿望不一致：主语+did eg: i wish i were you
与过去愿望不一致：主语+ had done/ could have done eg: i wish i had been to the US.
与将来愿望不一致：主语+ would/could do eg: i wish i could meet you tomorrow.
3. if it were not for.../ if it hadn't been for...要不是因为...，其后主句用虚拟
if it were not for your help, i wouldn't save her now（与现在事实相反）
if it hadn't been for your help, i wouldn't have saved her.（与过去事实相反）
Had it not been for your help, i would not have saved her.
4. lest（以防...）引导的从句，从句谓语用虚拟，即should do
They spoke in whispers lest they should be heard.
5. 再比如：would rather/ as if/ if only/ without/ otherwise...... in case/ for fear that
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