情態 (或語氣) 助動詞 (modal or modal auxiliary) can 的否定形式為 cannot，而 cannot 的縮寫為 can’t。反過來說，can’t 的原形是 cannot，不是 can not。Can not 的寫法不僅極為少用，而且被視為錯誤。在寫作或書面文件中，cannot 是個正式用字，而 can’t 並不適合用在這些場合；can’t 大多用於口語。再強調一次，cannot 要寫成一個字，不能寫成 can not。
例句：
- You should tell your son what he can and can not do! (你應該告訴你兒子他什麼能做什麼不能做！) (誤)
- We cannot be too careful in choosing friends. (我們在擇友時愈謹慎愈好 / 我們在擇友時無論多謹慎都不為過) (正) － 這一句是改寫自英國大文豪王爾德 (Oscar Wilde) 的 A man cannot be too careful in the choice of his enemies. (一個人在選擇敵人時愈謹慎愈好 / 一個人在選擇敵人時無論多謹慎都不為過)
Can not 現在僅被用於強調 not 的句子；問題是，喜歡用 can not (而不是 cannot) 來強調 NOT 的人還不在少數。不過，下句中的 can not 雖是兩個字，但卻是個完全正確、毫無爭議的句子，因為 can not 並非 can’t 的意思，這裡的 not 是 not only … but (also) … 這個相關字組的一部份：
- Norma can not only sing but play the saxophone too. (諾瑪不僅能唱而且也能吹奏薩克斯風)
茲將英文常見的縮寫字及其原形列表如下供大家參考：
縮寫 | 原形 |
aren’t | are not |
can’t | cannot |
couldn’t | could not |
didn’t | did not |
doesn’t | does not |
don’t | do not |
hadn’t | had not |
hasn’t | has not |
haven’t | have not |
he’d | he had, he would |
he’ll | he will, he shall |
he’s | he is, he has |
I’d | I had, I would |
I’ll | I will, I shall |
I’m | I am |
I’ve | I have |
isn’t | is not |
it’s | it is, it has |
let’s | let us |
mustn’t | must not |
shan’t | shall not |
she’d | she had, she would |
she’ll | she will, she shall |
she’s | she is, she has |
shouldn’t | should not |
that’s | that is, that has |
there’s | there is, there has |
they’d | they had, they would |
they’ll | they will, they shall |
they’re | they are |
they’ve | they have |
we’d | we had, we would |
we’re | we are |
we’ve | we have |
weren’t | were not |
what’ll | what will, what shall |
what’re | what are |
what’s | what is, what has |
what’ve | what have |
where’s | where is, where has |
who’d | who had, who would |
who’ll | who will, who shall |
who’re | who are |
who’s | who is, who has |
who’ve | who have |
won’t | will not |
wouldn’t | would not |
you’d | you had, you would |
you’ll | you will, you shall |
you’re | you are |
you’ve | you have |