May and might are usually associated with probability and possibility (present and past), although they can also express many other notions such as permission, suggestions, etc. In most cases they can be interchangeable, although might tends to sound a bit more formal than may. Some people also say that the probability or possibility expressed with might is more remote than that with may.
Here you will find some exercises combining different modals. Gook luck!
By the way, are you familiar with the expression "pigs might fly"? Have a look at this video by The Teacher (BBC Learning English) and you'll see.
