A 对现在事件的否定推断，可以用can't或couldn't加上动词be的现在式或者可以用can't/couldn't加上动词的进行式表示：

　　Child：Can I have some sweets?I'm hungry.

　　孩子：我可以吃些糖果吗?我饿了。

　　Mother：You can't/couldn't be hungry.You've just had dinner.

　　母亲：你不会饿的。你刚吃过饭。

　　Ann(looking through binoculars)：An aeroplane is pulling up people fromthe boat!

　　安(透过望远镜)：有一架飞机正把人们从小船吊到飞机上!

　　Tom：It can't/couldn't be an aeroplane.It must be a helicopter.

　　汤姆：不可能是一架飞机。一定是一架直升机。

　　—He says he's still reading The Turn of the Screw.

　　—He can't/couldn't still be reading it.I lent it to him ages ago and it's quite a short book.

　　—他说他还在读《螺丝在拧紧》。

　　—他不可能还在读那本书。我很久以前就把书借给他了，而且书也很薄。

　　B 对过去事件的否定推断，可以由can't/couldn't+动词的完成式或完成进行式表示：

　　—A man answered the phone.I suppose it was her husband.

　　—It can't/couldn't have been her husband.He's been deaf for ages.

　　—是一个男人接的电话。我想大概是她丈夫。

　　—不可能是她丈夫。他耳聋了许多年了。

　　—I took a Circle Line train to St Paul's.

　　—You can't/couldn't have taken the Circle Line.It doesn't go through St Paul's.You must have been on the Central Line.

　　—我乘地铁环线去了圣保罗大教堂。

　　—你不可能乘环形地铁。它不经过圣保罗大教堂。你一定是乘的中央线地铁。

　　如所表示的推断是过去做出来的或是由动词过去式引导的，就必须用couldn't：

　　She said I couldn't have come on the Cricle Line.

　　她说我不可能是乘地铁环线来的。

　　He said it couldn't be an areoplane.

　　他说那不可能是飞机。

　　否则can't或couldn't都可以使用。