Normally, yes. 'It' happens (should happen) every month. Except if a girl is pregnant. When a girl gets her period for the first times it can be irregular, but that's normal. Her body still has to...
umm yeah totally...you ovulate before you start you period. which means that there is a egg waiting to be fertilized. if its not fertilized then that's when you get your period. so your answer is...
at some point yes
Yes, usually. But if a girl happens to be a body builder or workout a lot, there's a chance her period will stop because of hormone changes and she will not be able to reproduce.