一般用法
if条件句（v.） 主句（v.） 与现在相反 did/were would/could/should/might+do 与过去相反 had done would/could/should/might+have done 与将来相反 did/were would/could/should/might+do were to do should(do)
变成一般疑问句时，可把现成助动词（be、情态、构将、构完）提前，再去掉if
特殊用法
语气(虚、实)的交错 You reminded me of the meeting, otherwise,I would have forgotten it. 真实 --（否则）-- 虚拟 虚拟：If you hadn't reminded me of the meeting,I would have......
时态上的交错 If I had worked hard before,my English would be good. 过去 现在
wish that+宾 与现在相反 did/were I wish I were a bird. I I 希望... 与过去相反had done I wish I had got up ealier. if only 与将来相反 do I wish I could live on the moon. 要是...就好了
It's (high) time that......是时候做...了 sb. did sth. sb. should do sth. =It's time for sb. to do sth.
as if/though 仿佛，好像 真实（ 可能是事实 ）It looks as if it's going to rain. 虚拟（不可能是事实）when a stick is in the glass with water, it looks as if it were broken. 与现在相反did/were 与过去相反had done
could have done 本可以做...