Fairly new to EDH and started asking questions not alot of people around me seemed certain about, so...
If my commander is destroyed, do I get to choose where it goes? And that it may stay in the graveyard or return to the commander zone?
(Kresh the Bloodbraided)And if my commander has died before and is now in my hand, is it 5 or 7 to cast?
Yes. If your commander is exiled/killed, you may have it go to the commander zone in lieu of the graveyard/removed from the game.
If it is bounced to your hand, you ignore the "commander tax" that occurs when it dies.
As an elaboration on funkalunk's post, the additional cost that applies when casting a general only applies when you cast it from the command zone.
You may also only choose to have your general return to the CZ if it would die or become exiled; under no other circumstances may you choose to do so.
If a commander would be put into the graveyard or exiled, you may send it to the command zone INSTEAD. If you do so then "dies" triggers and such will not trigger, although "leaves the battlefield" ones will.
If it is sent to your hand it stays, and you can cast it for its normal mana cost.
If it's sent to your library, hard luck (unless you can tutor it).
If a commander is exiled face down by any player, they MUST reveal it and send it back to the command zone.
"Commander tax" is applied each time your commander is cast from the command zone; it's equal to 2 times the number of times you have cast it from the CZ (casting from hand, yard, library, etc doesn't raise this number).
Finally, any "reduce the cost of the spell" effects (i.e. Animar, Soul of Elements) will eat into the commander tax, but things like Rooftop Storm will not (they only change the card's mana cost, not any additional costs).
So, If my commander gets bounced to my hand, and I later play it from my hand, does its "tax" reset?
Would it be considered a "new permanent" to such an extent that the tax resets because it went to my hand? (if it goes back to the CZ later)
No, the number of times your Commander has been sent to the command zone during a game is persistent. "Commanderness" is a property of the card, itself. Even though it's treated as a new game object each time it changes zones, the fact that it's your Commander card never changes.
No. The game essentially keeps a running total of how many times your commander/general has been returned to the command zone. Note that being a general is a property of the card itself, and the card may therefore be tracked in its movement between zones.