505.1. There are two main phases in a turn. In each turn, the first main phase (also known as the precombat main phase) and the second main phase (also known as the postcombat main phase) are separated by the combat phase (see rule 506, “Combat Phase”). The precombat and postcombat main phases are individually and collectively known as the main phase.
505.1a Only the first main phase of the turn is a precombat main phase. All other main phases are postcombat main phases. This includes the second main phase of a turn in which the combat phase has been skipped. It is also true of a turn in which an effect has caused an additional combat phase and an additional main phase to be created.
505.2. The main phase has no steps, so a main phase ends when all players pass in succession while the stack is empty. (See rule 500.2.)
505.3. First, but only if the players are playing an Archenemy game (see rule 904), the active player is the archenemy, and it’s the active player’s precombat main phase, the active player sets the top card of their scheme deck in motion (see rule 701.25). This turn-based action doesn’t use the stack.
505.4. Second, if the active player controls one or more Saga enchantments and it’s the active player’s precombat main phase, the active player puts a lore counter on each Saga they control. (See rule 714, “Saga Cards.”) This turn-based action doesn’t use the stack.
505.5. Third, the active player gets priority. (See rule 117, “Timing and Priority.”)
505.5a The main phase is the only phase in which a player can normally cast artifact, creature, enchantment, planeswalker, and sorcery spells. The active player may cast these spells.
505.5b During either main phase, the active player may play one land card from their hand if the stack is empty, if the player has priority, and if they haven’t played a land this turn (unless an effect states the player may play additional lands). This action doesn’t use the stack. Neither the land nor the action of playing the land is a spell or ability, so it can’t be countered, and players can’t respond to it with instants or activated abilities. (See rule 305, “Lands.”)