Elemental Resonance — ruling 1

You get the mana whether you want it or not. If you don't spend it, it will disappear at the end of the current step (or phase).

Elemental Resonance — ruling 2

If the enchanted permanent's mana cost is {2}{W}{U}, this Aura's controller adds {C}{C}{W}{U}. If its mana cost is {X}{G}, this Aura's controller adds {G}. If its mana cost is {W/U}{W/U}, this Aura's controller chooses whether to add {W}{W}, {W}{U}, or {U}{U}. The choice may be different each time the ability resolves.

Elemental Resonance — ruling 3

This ability isn't a mana ability. It uses the stack.

Elemental Resonance — ruling 4

Only the first main phase in a turn is the "precombat main phase," even if additional main phases are generated by some effect.

Elemental Resonance : règles, fonctionnement et interactions (MTG)

Elemental Resonance 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

Enchant permanent At the beginning of your first main phase, add mana equal to enchanted permanent's mana cost. (Mana cost includes color. If a mana symbol has multiple colors, choose one.)

How it works : Elemental Resonance

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
You get the mana whether you want it or not. If you don't spend it, it will disappear at the end of the current step (or phase).
No other card is named in this ruling.
#2May 1, 2006· Wizards of the Coast
If the enchanted permanent's mana cost is , this Aura's controller adds . If its mana cost is , this Aura's controller adds . If its mana cost is , this Aura's controller chooses whether to add , , or . The choice may be different each time the ability resolves.
No other card is named in this ruling.
#3May 1, 2006· Wizards of the Coast
This ability isn't a mana ability. It uses the stack.
No other card is named in this ruling.
#4May 1, 2006· Wizards of the Coast
Only the first main phase in a turn is the "precombat main phase," even if additional main phases are generated by some effect.
No other card is named in this ruling.