I'm completely new to Magic the Gathering. I've read the basic rulebook pretty thoroughly and have a pretty good grasp of the basic rules, but it's unclear to me as to when you can tap lands to generate mana.
First off, mana is supposed to dry up after each phase in a turn. Can I untap my land cards and then tap them again in the next phase if I need more mana to power a spell? Or does a land card remain tapped until my next turn like everything else?
Second, the rules mention that I can play instants and activate abilities during my opponent's turn. Does that then mean that I can tap my land cards during my opponent's turn to power those spells?
Third, does my opponent's untap step for their turn only apply to their cards? Or can I untap mine as well? Not closely related to land cards, but when I can tap and untap my land cards would make a huge difference as to how I play.
Hope these questions aren't painfully obvious, but I've just started playing the game. Thanks for your time!