Can you cast a spell with flash during your opponent's main phase before they play a land?

Flash (CR 702.8a) allows a player to cast a spell any time they could cast an instant, regardless of whether that spell is a sorcery, creature, or other type that would normally be restricted to main phases. Instants and spells with flash may be cast whenever a player has priority.

Priority is the key concept here. During your opponent's main phase, your opponent receives priority first when the phase begins (CR 117.3b). However, if your opponent passes priority without taking an action — or after resolving a spell or ability — priority passes to you. At that point, you may cast your flash spell (CR 117.3c).

The tricky part about land plays is that playing a land is a special action (CR 116.2b), not a spell or ability. Your opponent may play a land any time they have priority during their own main phase while the stack is empty. But if they haven't acted yet and simply pass priority to you (intentionally or by saying 'go' to a phase), you can cast your flash spell before they play a land that turn.

Example: It's your opponent's first main phase. They do nothing and pass priority. You cast Teferi's Ageless Insight (a flash enchantment). Your opponent cannot 'go back' and play a land before your spell resolves — they passed priority and you have acted.

Important caveat: if your opponent has NOT yet passed priority, you cannot simply interject and cast your spell. You must wait until priority is passed to you (CR 117.1).