① 虚拟现在时表示与现在事实相反的假设,其if 从句的谓语形式用动词的过去式(be 一般用were),主句用would/ should/ could/ might +动词原形,例如：If I were in your position I would marry her.
② 虚拟过去时是表示与过去事实相反的假设,if 从句的谓语形式用过去完成时即had＋过去分词,主句用would / should / could / might + have +过去分词,例如：If it had not rained so hard yesterday we could have played tennis.
③ 大多数的虚拟条件句属于上面三种情况的一种,但并不排除存在条件和后果中,一个和现在情况相反,另一个和过去情况相反,例如：If you had followed what the doctor said,you would not be so painful now.过去的选择对现在造成了后果.
还有一种因为违反常理而不会发生的事情,比如不日出,if句用过去时,主句用would加动词
