Ovulation Questions

Okay so I've been trying to understand ovulation and this is what I've gathered (PLEASE correct me if I'm wrong/help debunk these things):

It starts 14 days before your next period, no matter how long your cycles are

The egg, once released, can only last for up to 36 hours but may die as soon as 12

Once those 36 hours have passed it is basically impossible for you to get pregnant until your next period, simply because there is no egg for sperm to pair with

Sperm can live for up to 5 days so if you have sex during your period it's possible to get pregnant

Please help me understand this so I can practice safer sex!