凝咒杖

The creation of the copy and then the casting of the copy are both optional.

You don't pay the spell's mana cost. If the spell has X in its mana cost, X is 0. You do pay any additional costs for that spell. You can't use any alternative costs.

Spellbinder's second ability allows you to create a copy of the imprinted instant card in the Exile zone (that's where the imprinted instant card is) and then cast it without paying its mana cost.

Spellbinder imprints a card only once, when it enters. Attaching Spellbinder to a creature doesn't trigger its "enters" ability.

If the copied card is a split card, you choose which one side of it to cast, but you can't cast both sides. You can choose the other side later in the game, though.