my washing machine wont keep its belt on?

You don't say how old it is but if it is old the problem will probably be worn bearings. Although these generally have a good life span
Check this by firmly holding the rotor and moving it from side to side. It will move of course but if you feel metallic click when you move it the bearing will be worn.
It could be the belt has stretched but again these do tend to hold there shape.
If it is old I'm afraid it may be a case of renewal.

ian
November 2011