I don’t have a cell phone. If I had one, it would be convenient for me to get in touch with others.
If+had done，主语+would/might/should/could+have done
Anyone in his position would have done the same.
=If anyone had been in his position, he would have done the same.
If+were/did/(动词过去式)/should/were to do，主语+would/might/should/could+do
If it were to rain(=Should it rain=Rained) tomorrow, we would not go for a picnic.
if only引起的感叹句，相当于“How I wish+宾语从句”，意为“但愿……，要是……就好了”。
If only we students didn’t have so much homework!
If it were not for … (=Were it not for …)
If it hadn’t been for … (=Had it not been for …)
If it were not for your rich parents, you couldn’t live so easy a life.
but for, but that
but for+名词和but that+从句，意为“倘若不是，要不是”，接虚拟语气。
She could not have believed it, but that she saw it.
在suggest, proplse, insist, demand, require, request, order等表示建议、要求、命令的(动词后接的)名词性从句中需用虚拟语气，基本句型：主语+(should)+动词原形。
Mother insisted that John (should) go to bed at 9 o’clock.（宾语从句）
Many hospitals recommended that we use the letter DR ABC to remember what to do when giving first aid.（宾语从句）
It was required that children (should) be sent to school when they reach the age for school.（主语从句）
The suggestion that he (should) be invited was rejected.（同位语从句）
That is their demand that their wages (should) be increased.（表语从句）
it’s necessary/important that sb. should do…, should可以省略
It’s necessary that John take the exam first.
it’s strange/a pity/surprising that sb. should do…, should表示“竟然”
It’s a pity that she should miss the chance to go abroad.
How I wish we students had more free time to relax ourselves!
I failed in the maths exam. How I wish I hadn’t wasted so much time playing.
What a pity you can’t go to the party. How I wish I could dance with you at the party.
It’s (high/about) time that sb. did/should do …早就该…..
It’s high time that we did something to improve our environment.
would rather that sb. did …“宁愿…；更愿意…”，表示现在或将来的愿望；
would rather that sb. had done …“宁愿…；更愿意…”，表示过去的愿望。
I’d rather you posted the letter right away.
I’d rather that I hadn’t seen her.
as if(though)+主语+did/had done…表示过去的情况用过去完成时，表示现在与将来的情况用过去时。
Alan talked about Rome as if he had been there.