It depends.
If the Monster declares an attack or already attacked and it is switched to defense position by a card effect (like Enemy Controller or Book of Moon for example) the Monster's position cannot be changed by you, the controller, because it declared an attack already. A Monster's position cannot be changed by their controller for the rest of a turn after they declared attacked with it.
If the Monster did not attack yet this turn and their position is changed due to a card effect, it can be changed by the controller.
Unless there is a card effect preventing it, it can attack.
If the attacking monster is switched into Defence position, the attack ends. Damage Calculation is not entered, you do not compare the monsters' ATK or DEF.
Not manually, no. A monster that has declared an attack that turn, may not switch into defence mode. But it can be switched by the effect of other cards, including their own, in the case of Goblin Attack Force.
Yes if you wait a turn
You cannot tribute summon a monster in face up defense position. You can either tribute summon a monster in face down defense or face up attack position. If you want to summon in face up defense, you need to special summon.
No. When a monster you control attacks a monster with a Defense higher then the Attack of your monster, you take damage equal to the difference but neither monster is destroyed. The only exception is due to card effects such as Des Kangaroo.
Yes, that is allowed. You cannot, however, attack and then change to defense in the same turn.
The core rules state that a Normal Summon places the monster in face-up Attack position, and a Set places it in face-down Defense position. Special Summons are face-up Attack or Defense position, unless stated otherwise. If the continuous Trap card 'Light of Intervention' is active, monsters can be Normal Summoned in face-up Defense position.
Yes. Once per turn, a monster may switch from face-up attack mode to face-up defence mode, or vice-versa, as long as the monster was not summoned that turn, and had not declared an attack. If the monster is face-down, then turning it face up into attack mode is a Flip Summon, which can't be done on the turn the monster was Set from hand.
Yes, if you control a monster in defense mode and it is changed to attack mode during the Battle Phase by a card effect, it can attack. The "Zero Gravity" Trap card is an example of a card that can switch your monsters' battle position during the Battle Phase. But if the monster already attacked that turn, was switched to defense mode, then changed to attack mode again by an effect, it cannot attack again on that same turn.
Yes. If you hit a monster with higher defense than your attack, your attacking monster isn't destroyed, but you take damage. If the defending monster's defense is lower than your attack, then the defending monster is destroyed, but the defending player takes no damage, unless through an effect like "Dragon's Rage" or "Elemental Hero Flame Wingman"
The monster is flipped into face-up defense position and any battle damage that occurs is calculated. Afterwards the monster that was attacked remains on the field and is not sent to the Graveyard.Examples of monsters that cannot be destroyed in battle are Spirit Reaper and Marshmallon.If a monster attacks with an effect that enables it to deal damage equal to the difference in its ATK and the defending monster's DEF (like Twin-Sword Marauder or a monster equipped with Big Bang Shot), the damage is calculated but the monster is still not destroyed in battle.