Safehold Sentry — ruling 1

If the permanent is already untapped, you can't activate its {Q} ability. That's because you can't pay the "Untap this permanent" cost.

Safehold Sentry — ruling 2

If a creature with an {Q} ability hasn't been under your control since your most recent turn began, you can't activate that ability, unless the creature has haste.

Safehold Sentry — ruling 3

When you activate an {Q} ability, you untap the creature with that ability as a cost. The untap can't be responded to. (The actual ability can be responded to, of course.)

Safehold Sentry : règles, fonctionnement et interactions (MTG)

Safehold Sentry 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

, : This creature gets +0/+2 until end of turn. ( is the untap symbol.)

How it works : Safehold Sentry

Original English text — Wizards of the Coast / Scryfall

#1May 1, 2008· Wizards of the Coast
If the permanent is already untapped, you can't activate its ability. That's because you can't pay the "Untap this permanent" cost.
No other card is named in this ruling.
#2May 1, 2008· Wizards of the Coast
If a creature with an ability hasn't been under your control since your most recent turn began, you can't activate that ability, unless the creature has haste.
No other card is named in this ruling.
#3May 1, 2008· Wizards of the Coast
When you activate an ability, you untap the creature with that ability as a cost. The untap can't be responded to. (The actual ability can be responded to, of course.)
No other card is named in this ruling.