Savings plan and RIs for compute

Hi Team.

I have some questions about saving plans and reserved instances, I would like your help.

1.-When is recommended to use saving plans or reserved instances? how to decides?
2.-If I have a saving plan for 10 USD hourly commitment, to reach this 10 USD, the price for my compute usage is a "pay as you go", or this price includes discount? after that discount will be applied, only if my usage reaches the commit of 10 USD?
3.-If my compute usage is 09 USD in an hour, so it does not reach the hourly commitment (10 USD), will I pay 09 or 10 USD for that hour?
4.-Azure advisor is recommending to me, buy saving plan if I consider 30 look-back days, but if I consider 60 look-back days, Advisor does not show any recommendation appear around saving plans. So, how can I estimate and calculate the hourly commitment?
5.-If I have reserved instance, that in one moment matches with my VM, I understand the cost per hour will include reservation discount, but what happens if I change the VM size for another size does not match the reserved instance or the families/series, will I keep paying for the reserved instances, and additionally I will pay for VM as "pay as you go"?
I mean when I buy a reserved instance, I always will pay for it, it does not depend on if I have a matching VM or if I am using the reserved instance?

Thank you.