A 对现在事件的否定推断，可以用cant或couldnt加上动词be的现在式或者可以用cant/couldnt加上动词的进行式表示：
Child：Can I have some sweets?Im hungry.
孩子：我可以吃些糖果吗?我饿了。
Mother：You cant/couldnt be hungry.Youve 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 cant/couldnt 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 hes still reading The Turn of the Screw.
He cant/couldnt still be reading it.I lent it to him ages ago and its 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cant/couldnt+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 cant/couldnt have been her husband.Hes 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 Pauls.
You cant/couldnt have taken the Circle Line.It doesnt go through St Pauls.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couldnt：
She said I couldnt have come on the Cricle Line.
她说我不可能是乘地铁环线来的。
He said it couldnt be an areoplane.
他说那不可能是飞机。
否则cant或couldnt都可以使用。