May and Can

Both can and may can be used to talk about possibility. But there is some difference between them. Can is used to talk about theoretical possibility; may is used to talk about factual possibility.

Compare:

·         The road may be blocked due to the procession. (Factual possibility.) 

·         Any road can be blocked. (Theoretical possibility - It is possible to block any road.) 

·         There may be a strike next week. (It is possible that there will be a strike next week.) 

·         Strikes can happen any time. (It is possible for strikes to happen any time.) 

·         If you drive carelessly, you may have an accident. (Factual possibility) 

·         Accidents can happen any time. (Theoretical possibility) 

When we talk about possibility, could often means the same as may or might.

·         You may/might/could be right. 

May not and Cannot

May not suggests improbability. Cannot suggests impossibility.

Compare:

·         We may not go camping this summer. (= It is possible that we may not go camping.) 

·         We cannot go camping this summer. (= It is not possible for us to go camping this summer.)