Can you cast a spell after your opponent casts a spell on your turn before you get priority back?

When any player casts a spell, that spell goes on the stack. After it is placed on the stack, priority is passed. Crucially, the active player (the player whose turn it is) always receives priority first after any spell or ability is placed on the stack (CR 117.3b). So on your turn, after your opponent casts a spell in response to something, priority comes back to you before anyone else acts.

You cannot cast a spell or activate an ability in the middle of your opponent casting their spell. The act of casting — choosing modes, targets, paying costs — is a process (CR 601.2), and once it completes and the spell is on the stack, priority is awarded starting with the active player (CR 117.3b). Your opponent does not get to cast a second spell immediately after; they had to receive priority first to cast the first one.

So the flow is: you cast a spell → your opponent receives priority and casts their spell in response → the stack now has two spells → you receive priority again. At that point, you may cast another spell, activate an ability, or pass priority. You are never skipped.

Example: It's your turn and you cast Lightning Bolt targeting your opponent. Your opponent receives priority and casts Counterspell targeting your Bolt. The stack now has: Counterspell (top), Lightning Bolt (bottom). Priority returns to you — you can now cast another spell (like a second Bolt or a Dispel) before Counterspell resolves.

The key rules here are CR 117.3b (active player gets priority after a spell/ability is placed on the stack) and CR 601.2 (the process of casting a spell). You are never forced to "skip" your chance to respond on your own turn.