什么时候用CAN'T,什么时候用CANNOT

cannot与can not
依然是从English——The Real Deal那里看到的英语小应用：
Can的否定形式是什么?Can't.没错.如果不缩写是什么?Can not.错了.应该是cannot,一个词,而不是分开两个词.
这个错误我以前也犯过不计其数次.当然,我有借口--国内的课本里没教过.（至少我读初中的时候,老师绝对没有讲过这一点.）两年前我读newspaper reporting的课程时,每次稿子里的can not都被老师用红笔圈出来,我才意识到这是个错误.
为什么不可以搞混?简单地说,因为两者的意思完全不同.Cannot是“不可以”,而can not是“可以不”.举个例子：
I cannot believe this!（我无法相信!等于I can't believe this!）
I can not believe this!（我可以不相信!也就是说,I can believe this,or I can not believe this.我并不一定要相信,我可以选择不相信.）
在网上搜索了一下,找到这篇can not vs.cannot.由于我不可能给出更清楚、完整的解释,特摘抄TheMudge网友的原话如下：
What we should use when we mean "are unable to" or "are not permitted to" is one word:cannot.This is the word that we contract when we write "can't." It is a common error to separate "can" and "not." The American Heritage Dictionary is quite clear on this point:cannot is the negative form of the word can.
Here's one example I use:
If I say "I cannot smoke," I mean that I am unable to or not permitted to smoke.If I say,"I can not smoke," not modifies smoke.My meaning is:"I am able to not smoke," i.e.,I have the ability to avoid smoking,to prevent myself from smoking,or to stop smoking.
Another example:
If I am unable to go or am not permitted to go,I should write,"I cannot go." If I have a choice between going and not going,I may write,"I can go,or I can not go – it is my choice."
To all my fellow nitpickers out there:I am fully aware that the two-word construction "can not" is often used where the opposite of "can" is intended.Despite the ubiquity of this usage,it is still an error.
顺便提一句,我一直以为busses（bus的复数形式）是不怕麻烦的英国人的专利,怕懒的美国人一律拼成buses（就像用rumor,traveling来代替rumour,travelling等等）.几天前翻字典时无意间发现busses是美式拼法（比如这篇文章）.其实美国人两种拼法都用（buses比busses更普遍一些）,但其他的英语国家基本只用buses,所以busses就算是美语了.看来什么事都不能想当然.