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If an ability would produce one or more mana of an undefined type, it produces no mana instead.

Some abilities produce mana based on the type of mana another permanent or permanents “could produce.” The type of mana a permanent could produce at any time includes any type of mana that an ability of that permanent would produce if the ability were to resolve at that time, taking into account any applicable replacement effects in any possible order. Ignore whether any costs of the ability could or could not be paid. If that permanent wouldn’t produce any mana under these conditions, or no type of mana can be defined this way, there’s no type of mana it could produce.

To “tap [a permanent] for mana” is to activate a mana ability of that permanent that includes the {T} symbol in its activation cost. See rule 605, “Mana Abilities.”

An ability that triggers whenever a permanent “is tapped for mana” or is tapped for mana of a specified type triggers whenever such a mana ability resolves and produces mana or the specified type of mana.

A replacement effect that applies if a permanent “is tapped for mana” or tapped for mana of a specific type and/or amount modifies the mana production event while such an ability is resolving and producing mana or the specified type and/or amount of mana.

If an object’s enters-the-battlefield triggered ability or replacement effect refers to X, and the spell that became that object as it resolved had a value of X chosen for any of its costs, the value of X for that ability is the same as the value of X for that spell, although the value of X for that permanent is 0. This is an exception to rule 107.3i.

The tap symbol is {T}. The tap symbol in an activation cost means “Tap this permanent.” A permanent that’s already tapped can’t be tapped again to pay the cost. A creature’s activated ability with the tap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

The untap symbol is {Q}. The untap symbol in an activation cost means “Untap this permanent.” A permanent that’s already untapped can’t be untapped again to pay the cost. A creature’s activated ability with the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

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.

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.