情态动词是我们英语学习中很关键的一部分，也是考试的重点部分之一。那么我们应该如何更好的理解情态动词和懂得运用好情态动词呢。
情态动词 (Model Verbs) 又称为情态助动词 (Model Auxil-iaries), 主要有can, may, must, could, might, shall , should, will, would, ought to, need, dare等。情态动词与其他动词连用表示说话人的语气。情态动词可表达建议、要求、可能和意愿等。情态动词没有人称和数的变化。情态动词部分重点测试以下内容：
(1)情态动词+动词完成式: must have done
(2)情态动词+动词进行式: must be doing
(3)情态动词+动词完成进行式: must have been doing
(4)某些情态动词的特殊用法: need
一、情态动词+动词完成式
情态动词+动词完成式即“情态动词+ have + done分词”，表示对过去行为或动作进行推测、评论或判断。
1. must have done
must have done 表示对过去某事的肯定猜测。其否定或疑问形式都用can(could) 来表示.
例如： Since the road is wet, it must have rained last night.
He can’t have missed the way. I drew him a map.
“The dictionary has disappeared. Who could have taken it?”
当然对现在发生或将来发生的事情,要用must do 表示猜测, 否定为can’t do.
He must understand that we mean business.
You must be hungry after a long walk.
2.may / might have done
may / might have done 表示推测过去某事“也许”发生了.may 比 might 表示的可能性在说话人看来稍大些。例如：
I can’t find my keys. I may / might have left them at the school yesterday.
3. could have done 在肯定句型中, 往往用作委婉的批评. 本应该做什么,而没做; 有时也用作猜测.
You could have told us earlier.
Tom could have taken the dictionary.
4. ought to / should have done和 ought not to / shouldn’t have done
ought to / should have done 和 ought not to / shouldn’t have done 用于对已发生的情况表示“责备”、“不满”，分别表示“本应该…”和“本不应该…”。例如：
1)With all the work finished, I should have gone to the party last night.
2)You ought not to have made fun of him. He is not the one you laugh at but learn from.
5. needn’t have done
needn’t have done 表示过去做了某事，但没有做的必要， 意为“本没必要…”。例如：
You needn’t have waken me up; I don’t have to go to work today.
注：表示推测过去某动作发生的可能性时，就表示的可能性程度而言，must最大，could其次，may更次之，might最小。例如：
“I wonder how Tom knew about your past.”
“He must / could / may / might have heard of it from Mary.”
二、情态动词+动词进行式
情态动词+行为动词进行式(即情态动词+ be + doing形式)，表示推测或评论某动作现在是否正在进行。例如：
1)He must be playing basketball in the room.
2)She may be staying at home.