Can you copy a spell with split second on the stack?

Actually, this requires a nuanced answer: players cannot manually copy a spell with split second using activated abilities, but triggered abilities and replacement effects that copy spells can still function while split second is on the stack.

Split Second (CR 702.61) states that as long as a spell with split second is on the stack, players cannot cast other spells or activate abilities that aren't mana abilities. This stops players from, say, activating Isochron Scepter or using Twincast (a spell) in response — because those are activated abilities or spells respectively.

However, CR 702.61b explicitly clarifies that triggered abilities still trigger and resolve normally even while a split second spell is on the stack. So if you control a permanent with a triggered ability that says 'whenever a spell is cast, copy it,' that trigger would still go on the stack and copy the split second spell.

Concrete example: Your opponent casts Krosan Grip (which has split second). You cannot cast Twincast in response because Twincast is a spell. However, if you control Strionic Resonator... wait — that's an activated ability, also blocked. But if you had a triggered ability like Thousand-Year Storm, it would still trigger and copy the spell because triggered abilities are not restricted by split second.

In summary: players cannot use spells or activated abilities to copy a split second spell, but triggered abilities and replacement effects that copy spells are unaffected by split second (CR 702.61b).