Mana & costs
Core concepts
How you pay for things — generic vs. colored, and additional vs. alternative costs.
How it works
Mana comes in five colors plus colorless. A generic cost like {3} can be paid with any mana; a colored symbol like {R} must be paid with that color. Mana abilities produce mana immediately, without using the stack.
Costs can be modified: an additional cost is something you pay on top of the normal cost; an alternative cost replaces the normal mana cost. You can't pay two alternative costs for the same spell, and unused mana empties from your pool as each step and phase ends.
Common confusions
“Can I combine two alternative costs (e.g., two 'you may cast for' effects)?”
No. A spell can be cast for only one alternative cost. Additional costs, though, can stack on top of whatever base cost you're paying.
“Floating mana between spells.”
Mana stays in your pool until the current step or phase ends, then it empties. You don't lose life for leftover mana in modern rules.
“Generic vs. colorless.”
{2} is generic (any mana pays it). {C} specifically requires colorless mana — they aren't the same symbol.