A 對目前事件的否定判斷，可以用can't或couldn't加上動詞be的目前式或者可以用can't／couldn't加上動詞的進行式表示：

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

孩子：我可以吃些糖果嗎？我餓了。

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

媽媽：你不會餓的。你剛吃過飯。

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

安（透過望遠鏡）：有一架飛機正把大家從小船吊到飛機上！

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

湯姆：不可能是一架飛機。肯定是一架直升機。

He says he's still reading The Turn of the Screw^1．

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

他說他還在讀《螺絲在擰緊》。

他不可能還在讀那本書。我很長時間以前就把書借給他了，而且書也非常薄。

B 對過去事件的否定判斷，可以由can't／couldn't+動詞的完成式或完成進行式表示：

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．

是一個男性接的電話。我想大概是她老公。

不可能是她老公。他耳聾了很多年了。

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．

我乘地鐵環線去了圣保羅大教堂。

你不可能乘環形地鐵。它不經過圣保羅大教堂。你肯定是乘的中央線地鐵。

如所表示的判斷是過去做出來的或是由動詞過去式引導的，就需要用couldn't：

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

她說我不可能是乘地鐵環線來的。

He said it couldn't be an areoplane．

他說那不可能是飛機。

不然can't或couldn't都可以用。