Is it possible to play a land card when the stack is filled with unresolved spells and effects waiting to do their work? It may be useful since MTG came out with his new (at those times...) Landfall ability (Zendikar, if I remember well...). So, I question this because some years ago I was trying to counter a spell, but my opponent played a land card – to let some landfall ability to trigger, and get some advantages from this – and at the end of this stack he finally cast his spell. I lost the game some turns later. All I’m asking is already described above, but I ask it once again: Is it possible to play a land card when the stack is filled up?
No, not usually.
Both the rules (via a mechanism called Priority) and abilities (including spells) instruct you to play a land.
The normal way to play lands is through the permission given by priority. A player is only given permission to do this when the stack is empty during a main phase of their turn.[CR 117.1, 117.1c, 116.2a]
But priority is not the only thing that instructs you to play lands. There are spells and abilities (such as Brilliant Ultimatum) that instruct a player to play a card. The stack isn't empty when these spells and abilities have you play a card, and this is of no consequence.[CR 608.2c]
117.1. Unless a spell or ability is instructing a player to take an action, which player can take actions at any given time is determined by a system of priority. The player with priority may cast spells, activate abilities, and take special actions.
117.1c A player may take some special actions any time they have priority. A player may take other special actions during their main phase any time they have priority and the stack is empty. See rule 116, “Special Actions.”
116.2a Playing a land is a special action. To play a land, a player puts that land onto the battlefield from the zone it was in (usually that player’s hand). By default, a player can take this action only once during each of their turns. A player can take this action any time they have priority and the stack is empty during a main phase of their turn. See rule 305, “Lands.”
608.2c The controller of the spell or ability follows its instructions in the order written. [...]