1. I have read that the defending side can refuse to send any Defending units to Defend, but how about if the zone is burning? What i mean is, if i never send any of my units to defend and my unit can never be attacked? Correct me if i am wrong, please.

2. Also, If my zone is 7/8 on HP, and the attacking side want to attack that zone, can i once again not to send any unit to defend and let the zone be over-killed? what do the remending damages go to? let say if the attack side have 5 attack-point in total.

Can someone ask me please.

The defending player always has the choice which, if any, of the units that can defend actually do so, unless a card effect forces a unit to defend. If he prefers for some or all of them to not defend, that's just the way it is. It doesn't matter how much damage the zone can still take, or if it is already burning.

If the attacker deals more damage than it takes to burn the zone, the excess damage is lost, it doesn't spill over.

Which means my units can actually never be attacked if I don't send them to defend? unless there are some effects that focus my units to defend? Am I right?

Units can never be attacked, only zones (and legends) can be. But yes, if you don't want your units to defend, there's no way for the attacker to change that unless he uses a card effect that forces a unit to defend.

If you prefer to keep your units alive by not having them defend, that's fine, but you will take damage to your zones and, obviously, eventually lose that way. You also won't be dealing damage to the attacking units. It's a trade-off, what's more beneficial to you has to be determined anew every time you are being attacked. And your units aren't completely safe if they don't defend, there are after all other ways to remove them apart from combat damage.