In Dungeons & Dragons, most encounters with monsters are resolved through fighting. When combat begins, the actions that a player can do on their turn suddenly become finite. This means that a character can only do specific things in the space of one turn, all of which are kinds of actions broken into specific categories. A new player might think they can only run around and swing a sword, but there are many things that even a level 1 character can do when it's their turn.
Related: Dungeons & Dragons: New Player Guide To Fighters
The D&D Player's Handbook has an extensive section on combat and how actions work, but the basic rundown of what your character can do in a turn goes as follows:
When a fight begins in a D&D game, the most common thing that will happen is characters and enemies performing an Action. These involve everything from swinging a weapon, using magic, pouring a potion down an ally's throat, or cutting the cord holding up a rope bridge. A D&D character can only perform one Action per turn, unless stated otherwise, such as through the Fighter's level 2 Action Surge ability.
These are the most common things a player or an NPC will spend their Action on during their turn.
In D&D 5E, a character can move up to their maximum Speed during their turn, which is a number that's usually determined by their race. If the group uses a battle map, the squares on the map usually equal 5ft each, so if a character's maximum movement speed is 30ft, they can move six squares on the map.
If a character moves across an area classed as Difficult Terrain, such as wet mud or an icy surface, then it costs double the movement. This means that a character who usually has a Speed of 30ft can only move 15ft across that surface. Additionally, if a character is knocked prone or starts an encounter lying down, then it takes half of their Speed to get into a standing position.
A character can have different kinds of Speed unrelated to how they walk. The most common examples of this include climbing, swimming, and flying. Usually, a DM or player will put their character model on something (like a d6) to signify that their character is using an unusual form of movement and keeping a note of how far they've moved. Using multiple movement types in one turn is possible, but only up to your maximum Speed. This means a character with 30ft Speed can walk 15ft and then fly 15ft in the air.
It's possible to move in the middle of performing multi-stage actions. A typical example is a character who can perform two attacks in one turn through the Extra Attack class feature or if they're fighting with two weapons. They can move, hit, move, and hit again, so long as they don't exceed their Speed. Remember that this can potentially provoke Attacks of Opportunity from enemies, so be wary about moving in combat.
When combat has begun, you can move through an allied unit's space but not through an enemy space. This gives the party a bit more leeway in battle, at least in terms of movement. The biggest exception to this rule is the halfling, as the Halfling Nimbleness racial feature lets them move through any creature's space if they're a Medium or larger-sized creature. However, this can still provoke Attacks of Opportunity.
A D&D character can take a smaller, additional action in their turn, which can happen alongside an Action. These are called Bonus Actions, and they're clearly labeled as such in their description. If the rules of a spell or a special move don't mention if they're a Bonus Action, then they're most likely classed as an Action.
The first thing that should be mentioned out of the gate is that D&D spells have special rules for Bonus Actions! If a spell has a casting time of a Bonus Action, then the only other magic that can be used that turn is a Cantrip. If the character has cast a spell using their Action, then they cannot cast a Bonus Action spell in the same turn.
Here are a few of the more common uses of Bonus Actions during the average game.
Your character can take one Reaction during each of their turns. This kind of action happens as a direct response to something the opponent does, and it usually happens on their turn. There are some rare instances where a Reaction can happen on your turn, and the rules allow for multiple instances to happen as part of one big chain action. Once the Reaction happens, it interrupts whatever the opponent is doing, and they'll continue their original action if they can afterward.
The most common type of Reaction is the Attack of Opportunity. This gives the character a free attack if an opponent moves out of their melee range by more than 5ft. It's possible to circumvent this by moving around the enemy and staying within 5ft or using the Disengage Action, which allows the character to move without provoking an Attack of Opportunity.
A tiefling player can use their Reaction to cast Hellish Rebuke, which is another common use of Reactions. This allows the tiefling to counter an enemy attack with a blast of hellfire and is extremely useful in melee combat. The other common use of Reaction is the Counterspell spell, as this allows a caster to prevent an opponent's magic from working. Bear in mind that it's possible to counter Counterspell with a Counterspell, assuming both casters have Reactions, so your response can be shut down, and the enemy can continue their incantation.
Related: Dungeons & Dragons: Starting Tips For New Players
In most D&D combat encounters, players will be moving, performing an Action, and taking a Bonus Action, so knowing what your character is capable of in one round is vital to succeeding in battle. If the party faces a formidable opponent, you need to make the most of D&D's action economy and grind them down with as many moves as possible.