When exactly can I use the Untap effect in Magic the gathering?

Question

Im new to Magic and I was playing a card called Safehold Sentry

http://ww2.wizards.com/gatherer/CardDetails.aspx?id=147394

So I go to attack by tapping him and then I want to use his "Untap" ability right away to up his toughness. Can I do that? or do I wait for damage to be dealt first? If so...Whats the point of having that ability if you cant up your toughness before someone blocks me?

Also, Can I use the untap ability on my opponents turn?

Answer 1

You can untap a tapped permanent with an untap ability any time you could play an instant (unless it says otherwise).

You can tap an untapped permanent with a tap ability any time you could play an instant (unless it says otherwise).

You can't play an instant during the Untap step, the Clean Up step, or when a spell with split second is on the stack.

Safehold Sentry can use it's untap ability as soon as it is declared as an attacker, after a blocker has been declared, while damage is on the stack, or even during your opponents turn (for a surprise blocker with decent toughness).

Answer 2

The untap icon works pretty much the same as the tap icon. It's and activated ability that by default can be used anytime you can play an instant. It is also hindered by summoning sickness. As always, card text overrules general game rules.

Answer 3

I'm new to Magic the Gathering too and I have no idea ecause it's intricate your lucky to understand all the taps and untaps.