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.
No, if a monster attacks a defense position monster and is then changed to defense position by Kunai with Chain, it cannot be destroyed by battle. In this scenario, the attacking monster is no longer in attack position and thus does not have to compare its attack points to the defense points of the opposing monster. Instead, the battle is resolved in such a way that the attacking monster remains in defense position, and the battle does not result in destruction for either monster.
Yes if you wait a turn
In Yu-Gi-Oh!, when a Defense Position monster is destroyed, damage calculation does not occur. Instead, the monster is sent to the graveyard without involving damage calculation, as it is not in Attack Position. The player does not take any battle damage from the destruction of a Defense Position monster.
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.