If an opponent attacks a player or planeswalker but each creature attacking that player or planeswalker is no longer attacking that player or planeswalker by the time Tahngarth's ability resolves (most likely because they've left the battlefield), that player or planeswalker can't be chosen for Tahngarth to attack. If there's no player or planeswalker to choose this way but you still choose to give the opponent control of Tahngarth, Tahngarth isn't attacking.
Any effects that say that Tahngarth can't attack or can't attack a certain player (such as that of Propaganda, or if Tahngarth gains defender) affect only the declaration of attackers. They won't stop Tahngarth from becoming an attacking creature through its triggered ability.
If an opponent is attacking a planeswalker but not that planeswalker's controller, Tahngarth can't attack that player.
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Players can't take any actions between the time you choose to give control of Tahngarth to the attacking player and the time it becomes an attacking creature.
Commander damage is tracked for each commander. It doesn't matter who controls the commander as it deals damage. If Tahngarth deals 10 damage to one player while under your control and another 11 damage to that same player while under a third player's control, the player loses the game.
Tahngarth may end up attacking its owner this way.
Although Tahngarth is an attacking creature if it uses its triggered ability, it was never declared as an attacking creature. This means that abilities that trigger whenever a creature attacks won't trigger when it changes control and becomes attacking.
If Tahngarth gains menace, it can't be blocked at all.
Giving control of Tahngarth to an opponent doesn't cause it to become untapped. This means that Tahngarth may end up attacking several times between your turns.
