（一）情態(tài)動詞無人稱和數(shù)的變化，其后可跟不定式的一般式，也可跟不定式的完成式。can用在否定句中，表示否定的判斷或推測以及理論上的可能性，其肯定形式為must+原形；could+原形動詞表示現(xiàn)在或將來的推測或判斷。
That can’t be true.
It must be done.
You could be right.
（二）can和could可用于下列結構：
can’t help + v-ing                         
cannot help but不得不                    
can but只得
cannot choose but只得                
cannot…too (over)無論……也不過分
One cannot be too careful.
I can but say it again.

（三）can或could與不定式的完成式連用，表示說話人對過去發(fā)生的行為的真實性所持的態(tài)度（驚奇或懷疑）。can只用于疑問句或否定句。can’t/ couldn’t have + 過去分詞表示對過去情況的強烈否定推測，一般譯為“一定沒……”。
Where can she have gone?
She can’t have gone to school. — It’s Sunday.
He could have gone off with some friends.
（四）may和might均可表示可能性和許可。may可表示給予許可，might則不能。may用于現(xiàn)在和將來時，might用于過去時或表示婉轉意義，或可能性不大。
She may not be at home. (= It is possible that she is not at home.)
She can’t be at home. (= it is not possible that she is at home.)
Brown might come to sign up for the match 2 days later.
（五）may +不定式的完成式表示說話人對過去發(fā)生的事情的懷疑與推測；might+不定式的完成式表示對過去發(fā)生的事情可能性的推測，所推測的可能性有時與實際情況相反，一般含批評、責備的口吻�？赡苄宰钚�。
She is late. She may have got up late.
You might have done it better.

（六）must表示義務時，其否定式形式為don’t need to, needn’t + V或don’t have to.
You needn’t come to see me if you don’t want to.
You don’t have to talk so loud.
（七）must + 不定式的完成式表示對過去發(fā)生的事情較有把握的推測，譯為“一定……”，否定式為can’t have done.
The road is wet. It must have rained last night.
He can’t have finished all the exercises within an hour.
（八）ought to 與should + 不定式的完成式用于肯定句，表示該做而未做，用于否定句表示不該做的事而做了，有強烈的批評與責備口吻。
You ought to have handed in your homework this morning.
You should not have done such a foolish thing.

（九）needn’t have + 過去分詞表示做了不必做的事，譯為“其實沒有必要……”。
You have five days more. You needn’t have worked so hard.