核心问题
I can not see why the machine stopped working.《朗文英语正误词典》(p.61)将上面的句子列为病句,并说应将 can not 改为 can't 或 cannot。我认为 can not 像 could not, may not, will not 一样是正确的非缩略形式。Porter G. Perrin 教授的 Writer's Guide and Index to English 也说“cannot, can not 两者都可以用,但 cannot 更为常用。”
词典解答
我认为R.Quirk等人的解释能够给你满意的回答。他们说：拼作can not两个单词是很少见的，只有在希望否定主要动词，或特别强调否定词或希望把否定词分开时，才拼作can not：
Can you not interrupt, please?
Can I not help you?
He says we can manage when we can certainly not. (CGEL, § 3.39)