Does Counterspell counter a spell that has already resolved?

Counterspell requires a target spell on the stack. A spell exists as a spell only while it is on the stack (CR 112.1). Once a spell resolves, it either moves to the battlefield, graveyard, exile, or its owner's hand/library — at that point it is no longer a spell, so it cannot be targeted by Counterspell or any other counter effect.

Priority and the stack are the key concepts here. After a spell is cast, players receive priority and may respond before that spell resolves (CR 117.3b). Counterspell must be cast during this window — while the target spell is still on the stack — to have any effect. You cannot wait until after resolution and then try to counter it retroactively.

CR 509.2 explains that when a spell resolves, the game follows its instructions and it is then placed in the appropriate zone. CR 608.2 covers resolution in detail. Once that process is complete, the game moves on; there is no 'going back' to counter something already resolved.

Concrete example: Your opponent casts Lightning Bolt targeting your creature. You let it resolve and your creature dies. You cannot then cast Counterspell — Lightning Bolt is no longer on the stack and is sitting in the graveyard. Counterspell had to be cast before Lightning Bolt resolved to counter it.

If you want to stop a spell, you must act during the response window while it is still on the stack. Once resolution begins and completes, the moment to counter has passed permanently.