These are most the things that a player can do in a board game. At a high level, player abilities can either change the state of the game, or passively change the rules of a game.
Most player actions change the state of the game. For example, adding eggs to a card in Wingspan, or moving an invader from one area to another in Spirit Island. Whenever there is state in a booardgame, the players may be allowed to change it. Sometimes a boardgame designer may add state to a boardgame so that it can be adjusted by the players, for example, El Grande provides the player with little score markers for the different territories so that players can adjust the victory point values for a given territory. If these score markers were not provided, than the point values of territories would not be considered state.
To determine all of the state changes that a player can perform, write a full list of all the “state” that your game has, and all the ways that it may be adjusted. If tokens can be placed on locations, these tokens can be added, removed, or moved to a new location.
There are some ways to modify these basic state changes to create new abilities, which I will list here.
Instead of just changing state, an effect can depend on some existing game state. This can be a precondition, trigger, etc.
Usually the player who triggers an ability is afforded control over any choices related to said ability, but any player may be provided any particular choice on any given ability. For example, some powers in Wingspan give opponents the option to select a food from the birdfeeder. Changing which player makes a given choice creates a wildly different ability. This can be a cost of an ability or create a small minigame within the overall game.
The simplist way to modify state changes is to bundle multiple effects together such that multiple state changes occur from one ability. One interesting way to bundle multiple effects is to affect the active player and their opponents differently, like causing the active player to discard a card in order to hurt their opponents health.
Alternatively, the ability could provide the player with multiple possible effects, and either giving them the choice of which effect takes place or tying the outcome to some sort of condition. Similar to these, an effect may be capable of triggering multiple times based on some condition or player choice.
Effects have multiple degrees of necessity—how necessary is it that this particular effect is applied. There are four levels of requirement for abilities
An optional effect is self explanitory—the acting player may apply the effect if they so choose. These effects usually use the word “may” before describing the effect, like “you may remove one cube”.
A variable effect is similar. The acting player may apply the effect “up to” a certain amount. For example, “you may remove up to three cube”.
Effects that are necessary if possible are any effects that the player must implement, but, if they cannot due to the state of the game, there are no consequences. For example, some abilities in Wingspan require that the player discard a card at the end of their turn, but if the player has no cards left to discard at the end of their turn, there are no repurcusions for the player.
Necessary costs are effects that must be implemented or else the ability cannot be used or the remainder of the ability does not take place. There is often a second effect with the prerequisite of “If you do so”, referring to the previously stated effect. For example,
Discard one card. If you do so, your opponent must discard one card.
By default, effects should be necessary if possible. This reduces the writing necessary to identify effects as such, and is generally the most reasonable default.
The ability may come with some prerequisite condition to take place. This condition may identify some variable board state or fixed condition of the game. For example, an ability in Spirit Island may require that the target area not contain any towns—board state—or it may require that the target area be a forest—a fixed condition. Conditions can be combined with multiple effects to create all kinds of player abilities. For example, a condition can be used to select which of two effects take place. For example, Spirit Island contains a card that removes a blight if it’s available, and an invader if it’s not. A condition can also be used as the prerequisite to repeated applications of an ability, most commonly seen with “area-of-effect” abilities. An area-of-effect ability is basically, “apply this effect to all characters who satisfy the condition that they be inside said area.” Conditions may also be used to take away choice from the player. For example, “deal damage to the closest enemy” removes the choice of which enemy to target from the player and replaces it with a condition.