If I'm casting a spell that costs four mana, and I only have three lands, can I tap those three lands, use {Arbor Elf} to untap a forest, and then tap that forest again to pay for the spell?
Yes you can. As long as you don't change phases or steps..the mana doesn't empty from your pool. Resolved. 🔨
Really?
Yeah. It would really really really suck if not. Haha