I believe it’s generally known that “can” is the favored verb when asking about whether it’s physically or mentally possible to do a thing and “may” is favored when asking for permission, but there turn out to be some nuances that can be a little confusing. With the help of Bryan A. Garner’s A Dictionary of Modern American Usage (which I refer to often in these posts), I’ll outline some of the nuances.
First, Garner allows that in speech and informal writing, the distinction is insisted upon only by the “insufferable precisian.” So when blogging informally, feel free to use “can” in place of “may.” In fact, among three caveats Garner offers is one suggesting that over-use of “may” can give your writing a prissy tone.
The other two caveats:
- Use of phrases like “mayn’t I” or “may I not” can be stilted, so it’s fine to use “can’t I” in these cases, even when asking for permission.
- “Can’t” and “cannot” have come to be more common denials of permission than the equivalent “may” phrases. (I would add that using the “can” forms to deny permission seems more emphatic, since it progresses from “no, you may not” to “no, it is not humanly possible.”)
I find it a little curious that Garner says the distinction between the two words is “often advisable” but then lets us off the hook for the ways in which we commonly override the distinction anyway. As with any of these picky little distinctions in English, you’re likely damned by some portion of your readers no matter which choice you make, as the sticklers will scoff if you use “can” where “may” is (prissily) suitable, while readers more comfortable with casual writing may be put off by fussy insistence on “may.” Now, in any case, you’ve heard from the language expert (Garner, not me) what your options are and
can may will should <insert your verb of choice here> write accordingly.
If you’re interested in more on “may,” see my entry on the distinction between “may” and “might” here.