語言是一種擺盪在約定俗成與積非成是之間的產物。有時候看似鬆散，卻無形之中有一股力量在引導、規範。就以 cannot, can not, 與 can’t 之間的是是非非來看吧。
Collins COBUILD English Usage, 1992 HarperCollins Publishers 關於 cannot 的說明放在 can – could – be able to 這一項底下：
negative forms
The negative form of ‘can’ is cannot or can’t. Cannot is never written ‘can not’. The negative form of ‘could’ is could not or couldn’t. …
Concise Oxford English Dictionary, 11th Edition, 2004 在 cannot 裡有這樣的說明：
[Usage] Both the one-word form cannot and the two-word form can not are acceptable, but cannot is far more common. Reserve the two-word form for constructions in which not is part of a set phrase, such as ‘not only … but (also)’: Paul can not only sing well, he also paints brilliantly.
牛津大學出版社 (Oxford University Press) 在一九九六年出版的 The New Fowler’s Modern English Usage, third edition 由 R.W. Burchfield 修訂。(附註：Burchfield 是牛津大辭典 (OED) 補編本的主編，一九七二年到一九八六年間，主導編輯四本補編本，將 OED 從十三本擴充到十七本。) 這本用法書裡這樣寫著：
Cannot. This is normally written as one word (rather than can not) and is often pronounced like the reduced form can’t. One encounters can not occasionally in letters, examination scripts, etc.; the division seems more to do with custom ( ‘I have always written it this way’) than with emphasis. The reduced form can’t, which now seems to natural, is relatively recent in origin.
以上這些可以視為英國人的觀點。那麼美國人怎麼看呢？
我引用 Bryan A. Garner 編寫的 A Dictionary of Modern American Usage, OUP 1998 的說法 (Garner 在用法上的權威大致有英國 Fowler 的份量)：
cannot should not appear as two words, except in the rare instances when the not is part of another construction (such as not only … but also) … Cannot is preferable to can’t in formal writing.
再看另一本辭典 The New Oxford American Dictionary，OUP 2001，在 cannot 的說明裡寫著：
USAGE Both the one word form cannot and the two word form can not are acceptable, but cannot is far more common in all contexts. Indeed, can not has come to be so unusual that it may be read as an error. The two-word form is advised only in a construction in which not is part of a set phrase, such as ‘not only … but (also)’: Stevenson can not only sing well, but he paints brilliantly.
(以上的說法與 COED, 11 edition 很像，不排除有互相參考的可能。有趣的是，到了美國竟然被視為有「錯誤」的嫌疑。)
而在 can’t 的說明只有短短幾個字如下：contraction of cannot。
綜合以上英美兩國的資料來看， can not 分開寫的方式不是被視為錯誤，就是極為罕用。相反的，cannot 被認為是正式的使用方式。至於 can’t 一般只當成 cannot 的縮寫形式 (contraction)，而且是比較後來才出現的寫法，少有單獨討論。
關於 contraction 的使用原則，在上述 Bryan A. Garner 的用法書裡提到的一段話頗有參考價值：
Many writers, especially those who write in formal situations, feel uncomfortable with contractions. And perhaps contractions don’t generally belong in scholarly journals.
But why shouldn’t writers use them in most other types of writing? Some excellent writers use contractions to good effect, even in books.
選擇使用哪一種方式，端看使用場合，而沒有證據顯示英美國別的使用差異。