'may' and 'might' can sometimes mean the same thing. 'I may go' and 'I might go' can be equivalent. The difference between the two is very subtle. If you say 'I may go' it is more likely to happen than when you say 'I might go'. 'may' and 'can' are also sometimes used interchangeably. However, saying 'May I go to the restroom?' and 'Can I go to the restroom?' is saying two different things. In the first sentence, you are asking permission to, physically, go to the restroom and use it. In the second question you are questioning your ability to relieve yourself. Most people use these questions interchangeably and people understand what you mean to say. However, there are some people who will be more than happy to make it known that you have used the wrong word or simply remark, "I don't know can you?"