Q: What's the deal with X spells? Can I play my Genesis Wave, ____ Sun's Zenith?
A: The converted mana cost of a card is the sum of the numbers and mana symbols in the cost. Cards with X in the cost treat X as 0 when determining the converted mana cost of the card. Note that the word "card" is used, since the converted mana cost of an X spell (when the card is on the stack) is determined by what value you have chosen for X, plus all other numbers and mana symbols in the cost of the card.
A (short): count the symbols and numbers, treat X as 0. Did you count to 0, 1, 2, or 3? yes? you can play it.
Q: Split cards, do those work?
A: The comprehensive rules have been worded in such a way as to allow for some split cards to be played. If a split card's individual sections both have a converted mana cost less than 4, it is legal in the format. Split cards are still subject to the colour identity rules- so be sure that both halves are legal in your deck.
A (short): you can play: Far // Away, Turn // Burn, Night // Day, Dead // Gone, Life // Death, Stand // Deliver, Trial // Error, Rise // Fall, Fire // Ice, Give // Take, Hide // Seek, Illusion // Reality, Profit // Loss, Pure // Simple, Ready // Willing, Supply // Demand, Wear // Tear, Toil // Trouble, Wax // Wane