好几代的语法学家们和中学教师们主张can 只应用于表示做某事的能力， 同时may 必须用于表示许可。 因此，在学术上纠正其用法要求
The supervisor said that anyone who wants an extra day off may (not can ) have one, or
管理人说任何想要一日特别休假的人都可以得到（不是 can ） 一天， 或者
May (not can ) I take another week to submit the application?
我可以（不是 can ） 再花一周提交申请吗？
Only 21 percent of the Usage Panel accepts can in the latter sentence. But can has a long history of use by educated speakers to express permission, particularly in British English. What is more, the blurring of the line between can and may is socially and historically inevitable, since politeness often makes the use of can preferable in the “permission” sense. For example, the sentence You can borrow my car if you like is a more gracious offer than You may borrow my car; the first presumes the granting of permission, while the second makes a point of it. Still, it is understandable that insistence on the use of may should become a traditional schoolroom ritual, particularly in first-person requests such as
用法专题使用小组中只有21%的成员接纳can 用于后面一句中。 但can 被受过教育的说话者用于表示许可已有很长的历史， 尤其是在大不列颠英语中。而且，can 和 may 之间不明显的界限从社会和历史渊源上说也是不可避免的， 因为礼貌常使can 的使用比较适宜“允许”这个意义。 例如，句子如你想要的话，你可以借用我的车 是比 你可以借用我的车； 亲切得多的提议， 第一句假定表示许可，而第二句却限定于这一点。然而，主张may 的用法应成为课堂内的惯例也是可以理解的， 尤其是在第一人称中，如
May I leave the room?
我可以离开这屋吗？
since it requires the pupil to distinguish explicitly between what is possible and what is allowed, a difference not always apparent to younger children. And even in later life, observance of the distinction is often advisable in the interests of clarity. Thus, the sentence Students can take no more than three courses allows the possibility that a student who is unusually capable may take more, whereas Students may take no more than three courses does not. · The use of can to express permission is better tolerated in negative questions, as in Can't I have the car tonight? probably because the alternative contraction mayn't is felt to be awkward.
由于这要求学生清楚地区分可能的和得到许可的两者之间的差异，而这对小孩子们常常是不太明显的。并且在其今后的生活中，为明确起见，体会这种差异常常是很可取的。因此，句子学生们可以选修不超过三门的课程 给能力超常的学生学更多的课程提供了可能性， 然而学生们只能选修不超过三门的课程 则不能提供这种可能性。 can 的用法更可用于否定疑问句中表示许可， 如 我今晚不能使用这车吗？ ， 或许是由于可替代的缩写mayn't 感觉起来有些笨拙的缘故
参考资料：金山词霸