Timing using trap card

I don't understand the absolute rule of using trap card.
When it can use, or when it can't

The situation is
Opponent: have 1 monster in atk position with 1500 atk and 1 set card in their spell/trap zone
Mine: have 1 monster in atk position with 1750 atk and 3 set cards in spell/card zone (2 mirror wall and 1 reverse trap)

In my turn, i declair atk. Then the system ask me if i want to use any trap card or not. I refuse to use any. And then the opponent flip up metalmorph equiped to his 1500 atk monster. Then, the system ask me if i'm gonna use mirror wall or not, 2 times. But i don't want to use mirror wall. I want to use reverse trap but the system didn't allow me to. My question is why can't i use reverse trap.

Answer

Answer List

anoymous

Cards that modify ATK/DEF as well as Counter Traps can be used in the Damage Step. Note that Reverse Trap is NOT a card that modifies ATK/DEF, but rather a card that affects effects that modify ATK/DEF, hence it's excluded.

anoymous

Thanks for the answer. One more question please. How can i know which card can use in damage calculation step and which card can't.

anoymous

He activated metal morph during the damage step,so he had to use before the damage calculation for you to use it.It other words you missed the timing.Mirror Wall can be used during damage step.