Standard Actions are the main action type you will make use of on each turn. This comprises regular attacks, most special abilities, using items, and healing friends. The exhaustive list is as follows:

Attack
Activate an item (think drinking potions)
Heal or Stabilize an ally using a Healer's Kit, or yourself or an ally using a poultice.
Attempt to escape a grapple/restraints
Draw a concealed weapon
Attach/remove a modification to your weapon, like a grenade adapter or a bayonet.
Throw an explosive/place a trap
Defensive Posture - You gain +4 Armour Class and a +4 bonus to Agility saves until the beginning of your next turn or until you make an attack.
Use a Skill - Most skills require the use of a Standard action, such as Sneak to hide, Insight to examine an enemy, or Investigate or Perception to search an area.

Movement actions are so named because the main use of this action type is for moving around up to your movement speed. Things you can do with this action include:

Move (up to your movement speed)
Draw or sheathe a weapon
Pick up an item
Light a lantern
Move a heavy object (movement depends on weight)
Enter a vehicle/mount a mount (takes 15 feet of your movement speed)
Stand up from prone (takes 20 feet of your movement speed)
Stand up from kneeling
Ready or drop a shield
Take something out of your inventory
Reload firearms and crossbows
Movement Action abilities

If you take any of the above actions that include a reference to your movement speed, then you can use the rest of your Movement Action only to use the rest of your movement, not for anything else like reloading, accessing your inventory, etc.

Full actions represent a concerted use of effort, combining a Standard and Movement action into one bigger action. Appropriately, two of the main uses for Full actions are to do a Full Attack, and to Run.

Full Attack - Make double the number of attacks your weapon normally allows with a Standard action. If your weapon allows Burst Fire, this is a Full Attack for that weapon.
Run - Move double your movement speed and make no attacks.
Disengage - Move up to your movement speed, without provoking Attacks of Opportunity.
Use a Full Action ability
Fix a jammed or stuck weapon
Ready an action - choose any Standard Action, but set a condition for it to happen later (e.g. "I shoot when he steps through the door"), also using up your Reaction. You can move up to your Speed on your turn after you choose your Readied Action.

Bonus Actions are for abilities you gain during your adventure that specify that they use up your Bonus Action. You only get one Bonus Action on each of your turns, and in many cases you might just not use it at all. In some cases, an ability might allow you to do something normally covered under another action type, like making hiding with Sneak a Bonus Action, or giving you an extra attack with a second weapon you're wielding.

Bonus Action abilities

Free Actions are, as the name implies, free to take at any time during your turn (and in the case of talking, at any time). Unless otherwise specified, you can perform as many free actions as you'd like during your turn, at any time during your turn. Free Actions include:

Speaking
Drop an item
Go prone from standing/kneeling - being prone gives melee attackers Advantage against you, while Ranged attackers suffer Disadvantage.
Kneel from standing
Free Action abilities

Reactions, unlike other actions, happen outside your own turn. They represent brief interruptions in the actions of others, and most often are used for Attacks of Opportunity. As you progress, however, you might pick up abilities that make use of your Reaction, but you only ever have one per round. Reactions include:

Attacks of Opportunity
Readied actions (see above)
Reaction abilities

Attack Rolls and Modifiers

All attacks use a d20 as a base, but rules for roll modifiers are slightly different for melee and ranged weapons:
--All melee weapons add either your Physique or Agility modifier, depending on if the weapon has the Light property.
--Ranged weapons add your Perception modifier, with an additional negative or positive modifier inherent to the weapon itself.
--Ranged weapons in particular may have modifications, like scopes or improved sights, that add further bonuses to your attack roll.
--Attack rolls have to exceed Armour Class in order to hit: defenders win ties.

Damage Rolls and Modifiers

Each weapon has its own damage dice, and just like attack rolls, damage modifiers differ slightly between melee and ranged weapons:
--Melee weapons usually have smaller damage dice, but add the same stat modifier as your attack rolls to damage. This gives them a higher base damage that's more predictable.
--Ranged weapons use only their damage dice, which tend to be higher, giving them greater potential damage but more unpredictability.
--Bows are the exception to the above rule and add your Physique to damage rolls, but also have Physique requirements to wield like some melee weapons.

Critical Hits and Misses

Critical successes and failures outside of combat work exactly as you'd expect from D&D5e. In combat, they have additional effects and conditions.
--A critical hit can be achieved by rolling a 20 (or 19-20, depending on your weapon/perks), or by exceeding your target's Armour Class by 10 on your attack roll.
--When you get a critical hit, you roll your weapon's damage dice twice, but apply damage modifiers only once.
--Critical misses happen when you roll a 1 (or 1-2, depending on your weapon), and cause you to roll 1d100 on the Critical Miss table:

• 1-33: Misfire - Your firearm discharges improperly or jams. You cannot use it for ranged attacks until you use a Movement action to fix it. This can be on the same turn as the misfire.

  • Stuck - You put too much strength in, and got your weapon/fists stuck in or on something or caught by your enemy’s guard. You have to wrench yourself free as above.

• 34-66: Off-Balance - The recoil from your weapon disorients you or your swing goes wild, and the next attack against you gains Advantage.

• 67-100: Dropped - You drop your weapon on the ground. For unarmed attacks, roll damage against yourself as though you'd scored a normal hit.

Advantage and Disadvantage

These work exactly like in D&D5e but are more sparingly used: you roll two d20s, and use either the higher roll (Advantage) or lower roll (Disadvantage).

Number of Attacks

Except if increased with perks or other abilities or properties, all weapons have a base of one attack per Standard action. Firearms and crossbows have some special rules, however:
--Firearms and crossbows have a property called Fire Rate, which determines how many attacks they allow per Standard action (and doubled, for Full Actions). They are as follows:

• Manual - One shot per Standard action. Most firearms are Manual, from lever- and pump-action shotguns, to bolt-action rifles and single-action revolvers, as well as crossbows.

• Semi-auto - Two shots per Standard action.

• Full-auto - Two shots per Standard action, and Full Actions use Burst Fire rules.

Burst Fire

Firearms with a Full-auto fire rate are rare but very powerful, not least for their ability to do Burst Fire as a Full Action. This replaces regular Full Action attack rules.
--Instead of rolling to attack, roll your weapon's Burst Fire damage dice and determine the Agility save DC (10 + your Perception modifier + your weapon's Accuracy).
--Everyone in a cone out to your weapon's Range must roll an Agility save against this DC: successes take half damage, failures take full damage.
--Damage Reduction against your weapon's damage type is multiplied by the number of Burst Fire dice rolled.

Weapon Range

Every ranged and thrown weapon has a Range, incremented in feet. You can make attacks against targets beyond this, but all such attacks are made with Disadvantage.
--Thrown weapons have a Range equal to 20 + 10 x your Physique modifier.
--Using ranged attacks within 5 feet of your target imposes Disadvantage on your roll.
--Weapons with the Scattershot property roll half their damage dice beyond their range, rather than roll to attack with Disadvantage.

Attacks of Opportunity

These work exactly as in D&D5e: when an enemy moves out of the reach of your melee weapon, or vice versa, Attacks of Opportunity can be made using Reactions against the mover.
--Forced movement, like being shoved or falling, does not trigger Attacks of Opportunity. Compelled movement (i.e. being given a command to move if enthralled) does trigger them.
--Attacks of Opportunity can be avoided by taking the Disengage Full Action, which allows you to move up to your movement speed and not trigger reaction attacks.
--Some other abilities and perks may offer Attacks of Opportunity in other situations.

Shoving and Grappling

These also work exactly like in D&D5e, replacing an attack you make with a Standard or Full action. If you get multiple attacks with a Standard action, this replaces just one of them.
--Unlike D&D5e, if you get a critical hit while rolling to Shove, you move your target 10 feet away rather than 5 feet.
--Both Shoves and Grapples add your choice of your Physique or Agility modifier to the d20 roll. For purposes of Advantage, treat these as melee attacks.

Improvised and Non-proficient Weapons

You can still attempt to attack with a simple or martial weapon even if you are not proficient in it. Objects not under one of these two categories are considered Improvised.
--Non-proficient and improvised weapon attacks are made without any modifier to the attack roll, even if the weapon has an inherent attack modifier.
--Improvised weapons include shattered bottles, chairs, and ranged weapons used for melee or thrown attacks, particularly gun bashing.
--Melee weapons that are thrown but lack the Thrown weapon property are also considered improvised for these rules.
--Improvised weapon attacks deal 1d4 damage + your Physique modifier. Melee weapons used in an improvised fashion keep their damage dice.

Using Explosives

Explosives - from petrol bombs to high-yield cluster grenades - are not used like regular ranged or throwing attacks. Instead, they're more like area-of-effect spells in the vein of D&D.
--Thrown explosives, like grenades, have a specific throwing range and blast radius. You select a space within this range, rather than a person or creature as the target.
--All those in the blast radius must make an Agility save against your Intelligence; success halves damage, while failure takes full damage.
--A rare few weapons, like rocket launchers, make normal attack rolls against a single target: if they hit, the target takes full damage and those in a radius take half.

Using Poisons

Poisons are the weapon of choice of expert assassins and venomous animals. You may fall into either or neither of these categories, but poisons can help you all the same.
--Poisons can be either found in your travels (in vials or powders), or they can be synthesised from natural materials using a Poisoner's Kit.
--Found poisons will list their saving throw DC, application method (contact or ingested), damage, and additional effects varying for each poison.
--Synthesised poisons can be either generic, or can be based off poison you've harvested from an identified poison plant or venomous creature.
--Generic poisons deal 1d8 poison damage and have a saving throw DC equal to 12 + your Intelligence modifier, and inflict the Poisoned status effect.
--Contact poisons can be applied to one melee or throwing weapon or 5 pieces of non-firearm ammunition as a Bonus Action, and last for 1 minute or until applied on a hit.

Traps!

The world is a dangerous place, and you will certainly encounter traps. Or, you can set up traps yourself to confound your enemies.
--Traps have effects and damage listed in their part of the Weapons table, see Items and Equipment.
--Traps are triggered variously through being stepped on, via tripwires, sensors, or opening a door or container.
--Depending on the type of trigger, you can notice or avoid traps with a Perception check, or with an Agility saving throw.
--Some traps, like mines, deal area-of-effect damage: whoever triggered the trap takes full damage, and those in the damage radius make Agility saves to take half.
--If you move onto a trapped space, you make an Agility save; if you succeed the trap is not triggered and you are able to see it.

Cover

When you are behind a solid object (not a person!), depending on how much of your body is concealed by it, you gain the benefits of cover against ranged attacks from the opposite side.
--Half Cover grants +2 to Armour Class and to Agility saving throws. Half Cover reaches to about your stomach or just below your chest.
--Three-Quarters Cover grants +5 to Armour Class and to Agility saving throws. Three-Quarters Cover is up to your chin.
--Full Cover makes you unable to be targeted with ranged attacks from the other side of cover.

Damage Types

There are three main types of damage that weapons deal: piercing, slashing, and bludgeoning. Other less common damage types include fire, explosive, poison, acid, and energy.

• Piercing - The most common and commonly resisted damage type.

• Slashing - The second most common damage type; armour that defends against piercing damage usually does not offer protection against slashing damage.

• Bludgeoning - The least common main damage type, most effective against all armour and most objects.

• Fire - A common damage type, can lead to burning and is effective against objects.

• Explosive - The damage type of explosives, a mix of piercing and fire - enemies resistant to those will usually also have some Explosive resistance.

• Poison - Usually found as a secondary damage type, like from animal bites or poisoned weapons. Bypasses armour and inflicts the Poisoned status effect.

• Acid - A rare damage type that can corrode weapons and armour, reducing Damage Reduction.

• Energy - The rarest damage type, applying not only to advanced laser weaponry but also to some things like lightning strikes.

Damage Reduction

Some outfits (and some monsters' hides) offer a level of protection above and beyond Armour Class, and outright reduce damage taken from all but specific damage types.
--Damage Reduction is expressed like so: [5/pi-s-f-ex]. This denotes armour that deducts 5 points from piercing, slashing, fire, and explosive damage.
--In the specific case of explosive damage, Damage Reduction is multiplied based on the number of explosive damage dice rolled.
--Similar rules apply for Burst attacks and attacks made with shotguns and similar multi-shot weapons.

Protection and Vulnerability

Alongside Damage Reduction, there are also Resistances and Vulnerabilities. They always affect only one damage type each.
--Resistance means incoming damage of a certain type is reduced by half, rounded down.
--Vulnerability means incoming damage of a certain type is doubled.
--Resistance/Vulnerability is calculated after Damage Reduction: if an attack deals 6 damage and the target has 3 Damage Reduction and Resistance, the result will be 1 damage taken.