Can you cast a sorcery during your main phase if your opponent has a spell on the stack?

Casting a sorcery requires meeting the conditions for casting a spell 'at sorcery speed.' Under CR 307.1, a sorcery can only be cast during your own main phase when the stack is empty and you have priority. If your opponent has a spell on the stack, the stack is not empty, so this condition is not met.

Even though it is still technically your main phase when your opponent responds to something you did, the presence of any spell or ability on the stack disqualifies you from casting sorceries. Both players must pass priority with an empty stack for the game to proceed to the next step or phase.

Priority also matters here: after your opponent puts their spell on the stack, they pass priority to you (CR 116.3). You do have priority in your main phase at that moment, but the stack is not empty, so sorcery-speed casting is still forbidden.

Concrete example: You cast Lightning Bolt during your main phase. Your opponent responds by casting Counterspell. You now hold priority with Counterspell on the stack. Even though it is your main phase, you cannot cast Divination (a sorcery) right now because the stack is not empty. You would need to let the stack resolve fully before casting it.

Some cards or abilities grant exceptions — for example, cards with flash (CR 702.34) or effects that say 'you may cast this as though it had flash' — but absent such an exception, the empty-stack rule is absolute for sorceries.