情态动词 （Model Verbs） 又称为情态助动词 （Model Auxil-iaries）。英语中助动词主要有两类：一是基本助动词，如have， do， be；二是情态助动词，如may， must， need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。在大学英语四、六级考试中，情态动词部分重点测试以下内容：

　　（1）情态动词+行为动词完成式

　　（2）情态动词+行为动词进行式

　　（3）情态动词+行为动词完成进行式

　　（4）某些情态动词的特殊用法

　　一、情态动词+行为动词完成式

　　情态动词+行为动词完成式即“情态动词+ have + v-ed分词”，表示对过去行为或动作进行推测、评论或判断。

　　1. must have v-ed

　　must have v-ed 表示推测过去某事“一定”发生了。其否定形式为：can't / couldn't have v-ed， 表示过去不可能发生某事。例如：

　　1）Since the ditch is full of water， it must have rained last night.

　　2）You couldn't have met my grandmother. She died before you were born.

　　2. could have v-ed

　　could have v-ed 表示推测过去某动作“很可能”发生了。例如：

　　1）“The dictionary has disappeared. Who could have taken it？”

　　2）“Tom could have taken it. He was alone yesterday.”

　　3.may / might have v-ed

　　may / might have v-ed 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.

　　4. ought to / should have v-ed 和 ought not to / shouldn't have v-ed

　　ought to / should have v-ed 和 ought not to / shouldn't have v-ed 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 make fun of him. He is not the one you laugh at but learn from.