Question about GB and Negate Attack (Yugioh)?

If your opponent has already declared his attack then chaining Threatening Roar onto it will stop any future attacks but not the one that has already been declared.

You need to use Threatening Roar either in your opponent's Main Phase 1 or the start of their battle phase before any declarations are made. If you don't then the first attack will happen as normal and the GB that attacked will be able to tag out at the end of the battle phase.

Negate attack however is only usable when your opponent declares an attack and will negate the attack in question and end the battle phase.

Threatening Roar is better as it is chainable onto nearly anything, just a matter of making sure you get the timing right.

Tenori is kinda wrong.

When you activate Threatening Roar, that means he's stopping the attack, not future attacks. That means that your assuming to activate it before he attacks. I know, that sounds wrong, but that's how people play it in tournaments and people use it like that. If there was a ruling against Threatening Roar, then it wouldn't have been that popular to be used, but there isn't any ruling against that.

Aside from that, the Glad Beast can't use his effect, cause Threatening Roar makes no battle phase, so the Glad Beast never really attacked.

Negate Attack negates the attack, so that would also mean the monster never really attacked.