From childhood on, we have a tendency of confusing these two modal verbs, each used with main verbs to denote a specific context of that main action:

  • may expresses permission or possibility
  • can expresses ability

While we can often get away with the mistaken uses of these words in speech, always check them when you are revising your document.

May for permission or possibility
May has two different meanings when used with a main verb, as illustrated below.

My manager replied to my email with a terse refusal: “You may not take this afternoon off. Sorry.”
(This use of may denotes permission, or in this case a denial of permission!)

According to the forecast, it may rain tonight.
(In this context, may denotes likelihood or possibility.)

Can for ability
Can denotes the ability to do something, as these examples illustrate.

The employees in Tim’s group can be very resourceful when they need to be.

Camels can go great distances without water.

The system can fail under these circumstances.
(Notice the difference in meaning between the sentence above and this one.)

The system may fail under these circumstances.
(In the first sentence, the system is capable of failing. In the second, there’s a distinct chance that it will.)