Hydra Trainer — ruling 1

If an exerted permanent is already untapped during your next untap step (most likely because an effect untapped it), exert's effect preventing it from untapping expires without having done anything.

Hydra Trainer — ruling 2

If you gain control of another player's permanent until end of turn and exert it, and then that player regains control of it, it will untap during that player's untap step.

Hydra Trainer — ruling 3

You can exert Hydra Trainer as you declare it as an attacking creature. You can't do so later in combat, and creatures put onto the battlefield attacking can't be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.

Hydra Trainer — ruling 4

You don't choose a target for Hydra Trainer's first ability at the time it triggers. Rather, a second "reflexive" ability triggers when you exert Hydra Trainer this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.

Hydra Trainer — ruling 5

The value of X is determined only once, as the reflexive triggered ability resolves.

Hydra Trainer — ruling 6

You can always activate an ability that will cause a creature to adapt. As that ability resolves, if the creature has a +1/+1 counter on it for any reason, you simply won't put any +1/+1 counters on it.

Hydra Trainer — ruling 7

If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.

Hydra Trainer : règles, fonctionnement et interactions (MTG)

Hydra Trainer is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

You may exert this creature as it attacks. When you do, target creature gets +X/+X until end of turn, where X is the number of counters on permanents you control. (An exerted creature won't untap during your next untap step.) : Adapt 2. (If this creature has no +1/+1 counters on it, put two +1/+1 counters on it.)

How it works : Hydra Trainer

Original English text — Wizards of the Coast / Scryfall

#1June 7, 2024· Wizards of the Coast
If an exerted permanent is already untapped during your next untap step (most likely because an effect untapped it), exert's effect preventing it from untapping expires without having done anything.
No other card is named in this ruling.
#2June 7, 2024· Wizards of the Coast
If you gain control of another player's permanent until end of turn and exert it, and then that player regains control of it, it will untap during that player's untap step.
No other card is named in this ruling.
#3June 7, 2024· Wizards of the Coast
You can exert Hydra Trainer as you declare it as an attacking creature. You can't do so later in combat, and creatures put onto the battlefield attacking can't be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.
No other card is named in this ruling.
#4June 7, 2024· Wizards of the Coast
You don't choose a target for Hydra Trainer's first ability at the time it triggers. Rather, a second "reflexive" ability triggers when you exert Hydra Trainer this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.
No other card is named in this ruling.
#5June 7, 2024· Wizards of the Coast
The value of X is determined only once, as the reflexive triggered ability resolves.
No other card is named in this ruling.
#6June 7, 2024· Wizards of the Coast
You can always activate an ability that will cause a creature to adapt. As that ability resolves, if the creature has a +1/+1 counter on it for any reason, you simply won't put any +1/+1 counters on it.
No other card is named in this ruling.
#7June 7, 2024· Wizards of the Coast
If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.
No other card is named in this ruling.