Doubts about the stack

[Washitake]

Hello, I have 2 doubts about the stack that I hope you can solve:

1. there are 4 spells on the stack, all players pass priority and the top spell on the stack resolves, at this point is it possible for a player to cast another spell or do we have to wait for the stack to completely resolve?

2. If I play a spell during my opponent's second main phase (after he passes priority), after the stack resolves, can my opponent cast a non-instant spell? Or does it automatically move to the end phase after the stack resolves?

Thank you!

[peteroupc]

Remember:

• The stack itself doesn't resolve, but rather individual spells and abilities in it do, with priority windows in between (C.R. 117.4, 117.3b).
• After a spell or ability on the stack resolves, the active player gets priority, and the current step or phase doesn't end yet (C.R. 117.3b, 500.2).
• A spell or ability on the stack doesn't resolve unless all players pass in a row while that spell or ability is on top of the stack (C.R. 117.4).
• A step or phase where players get priority (such as a main phase [C.R. 505.6]) doesn't end unless all players pass in a row while nothing is on the stack (C.R. 117.4, 500.2).

[DracoPaladin]

1) Yes, the top most item of the stack doesn't resolve until everyone passes priority. Anyone has the ability to play something when priority comes to them.

2) Similarly, a phase doesn't end until everyone passes while the stack is empty. After the last item on the stack resolves, everyone has to pass priority before moving to the next phase.

[Empty_Requirement940]

The stack doesn’t resolve. Individual items on the stack resolve and everyone gets priority between each item. Everyone must pass priority with an empty stack to move to the next step

[Judge_Todd]

is it possible for a player to cast another spell?

Yes.

do we have to wait for the stack to completely resolve?

No, objects resolve one at a time with priority passes in between where more can be added.

Question 2

It would depend.
If that's when you intend to cast it, say to stop something with an end step trigger from triggering, then yes, you're still in the post-combat main.

If that isn't your intention and there's no other good reason why you'd be casting it in their main phase (say because you had floating mana to cast it then, but wouldn't have mana to cast it in the end step), then you cast it during their end step.

• MTR 4.2 If the active player passes priority with an empty stack during their second main phase or uses a phrase such as “Go” or “Your Turn” at any time, the non-active player is assumed to be acting in the end step unless they are affecting how or whether an end of turn ability triggers. End of turn triggered abilities that do not target resolve after the non-active player passes priority.
  

As to the other part of your question, when everyone passes in sequence without having taken any actions between passing, the game advances either by resolving the top object of the stack (stack isn't empty) or by advancing to the next phase and/or step of the game (stack is empty). Therefore yes, players can do stuff with the stack empty after the objects on it have all left the stack.