Help understanding Duel Masters

I am hoping to pick the brains of people who are in the know because this game is somewhat confusing.

  1. when I put a card in my “mana zone” does that mean I cannot ever use if for that fight?

  2. is there an advantage to putting one card or another (of the same color) into the mana pile? I mean if both a 5000 strength blocker and a 1000 point spell put in the same amount of mana why would I ever put in the expensive cards?

  3. Can someone explain battles a bit? Last night for example I had three monsters summoned but I never got the opportunity to attack. Then suddenly I could…I think I can only attack if one of the enemy monsters is already tapped but I am not sure. So any insight would be great.