One word is often misused for the other.

Problem:
The auxiliary verbs “can” and “may” are not synonyms, at least not in their primary definitions.

Explanation:
The primary definition of the auxiliary verb “can” is to be able to. For example, He can dislocate his own shoulders. would be an appropriate use of this auxiliary verb. The primary definition of the auxiliary verb “may” is to be allowed or permitted to. For example, May I have another piece of cake? Yes, you may. would represent appropriate uses of this auxiliary verb.

Unfortunately, many use “can” when they should use “may”. I believe that the use of “can” instead of “may” when the latter is appropriate represents

• a laziness that favors easier-to-pronounce words,
• a foreign-language influence, and
• an aversion by Americans to ask for permission.

The word “can” is easier than the word “may” to say. “Can” is spoken with an open mouth and comes out of the mouth quickly. It also begins with a hard consonant, which is easier than a soft consonant to vocalize. “May” requires the speaker to begin with a closed mouth and a soft biting of the lips, so it starts with a soft consonant and ends with a vowel sound. “May” must be vocalized longer than “can” to be understood. Try to say “may” quickly; it will confuse many listeners.

Also, consider the influence on American English today of a relatively large population of native-Spanish speakers. The Spanish auxiliary verb “poder” truly means both “can” and “may”; Puedo usar el baño. means both I can use the bathroom. and I may use the bathroom.

As more native-Spanish speakers in the U.S. learn English, they will tend to use “can” instead of “may” because “can” is more easily pronounced and because “can” and “may” are interchangeable in their native language.

Finally — here comes the psychological mumbo-jumbo part — I believe that Americans in general (including me) have an aversion to asking for permission.

So we prefer “Can I have another piece of candy?” to “May I have another piece of candy?” because semi-consciously or unconsciously we believe that we’re not asking for permission by using “can” instead of “may”.

Solution:
Use “can” when you want to refer to ability, not permission; use “may” when you want to refer to permission.