A 对目前事件的否定判断，可以用can't或couldn't加上动词be的目前式或者可以用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．
妈妈：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）：有一架飞机正把大家从小船吊到飞机上！
Tom：It can't／couldn't be an aeroplane．It must be a helicopter．
汤姆：不可能是一架飞机。肯定是一架直升机。
He says he's still reading The Turn of the Screw1．
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。我很长时间以前就把书借给他了，而且书也非常薄。
B 对过去事件的否定判断，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．
是一个男性接的电话。我想大概是她老公。
不可能是她老公。他耳聋了很多年了。
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。你肯定是乘的中央线地铁。
如所表示的判断是过去做出来的或是由动词过去式引导的，就需要用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。
不然can't或couldn't都可以用。