一、概念

它是一种动词形式，表示说话人的某种假设、愿望、怀疑、猜测、建议等含义。


二、语法结构
  1.if结构（非真实条件句——表示的是假设的或实际可能性不大的情况）
   
        与..事实相反        If从句                                     主句
        过去                 Had done                             Would* have done
        现在                 Were/did                               Would* do
        将来                 Should do/were/were to do   Would* do

        例句：

      If there hadn’t been much rain in spring, we would have had a good harvest now.  [过去] 
       If he smoked less, his cough might be well soon.  [现在] 
       If the lecturer should be late, you would have to make a speech first.  [将来]

       备注：
       （1）上表中’*’，would都可转换为should、could、might。  
       （2) 如果为时间错综句的话，左右两栏可互相搭配，排列组合。
        例句：
        He would still be alive today if he hadn’t taken that drug.
                         [与现在事实相反]      [与过去事实相反]

        (3)  if可转换为其他形式
      例句：Without your advice, our meeting wouldn’t have been so successfully.       [过去]
       （=>可转换为：If there hadn’t been your advice, we …）
       Your notes are almost illegible (难以辨认的). Notes typed out would be a lot easier to  read.     [将来]
        (=> 可转换为：If the notes had been typed out, it would be a lot easier to read.)
         I should have gone to see Dr. Smith and he might have cured me of the disease.[过去] 
         (=> 可转换为：If he have gone to see Dr. Smith, he ..)