Rules

Understand creature turns: Each turn has a move action (move your full distance, up to 30' for most, or take other move-specified actions; moving 5' doesn't consume the move action) and a standard action (make one weapon attack or cast most spells; or take other standard-specified actions). You can choose to take a second move action instead of a standard action.

Utilize full-round actions: If you do not move, you can take a full-round action to make multiple weapon attacks (if able) or cast a spell requiring a full-round action (like most summoning spells), or take other full-round specified actions.

Use swift actions: Certain abilities, like casting a quickened spell, require a swift action. You can only perform one swift action per turn, but it doesn't prevent a full-round action.

Perform free actions: Abilities like a Barbarian's Rage are free actions, taking no time, and you can perform as many as you like per turn.

Calculate Attack Bonus (AB): Roll 1d20 and add your AB. AB is determined by Base Attack Bonus (BAB) (0 to +20 for a 20th level Fighter), plus modifiers from Strength (+1 per 2 points above 10 for melee), Dexterity (replaces Strength for ranged/certain weapons with Weapon Finesse), flanking (+2), enchantment bonuses (e.g., +1 weapon), feats (e.g., Greater Weapon Focus), morale bonuses (e.g., Potion of Heroism), and other bonuses (luck, competence).

Check against Armor Class (AC): If the modified 1d20 roll is greater than or equal to the opponent's AC, you hit. A roll of 1 is always a miss, and a 20 is always a hit (potentially a critical hit).

Understand AC sources: AC includes Base (10), Dodge bonuses (from attributes like Dexterity and feats), Size (+1 for gnomes/halflings), Armor (light, medium, heavy), Shields (or shield AC from feats), Natural bonuses (e.g., Snakeskin), Deflection bonuses (e.g., Ring of Protection), and penalties from negative statuses (e.g., cowering). Most bonuses of the same type do not stack, except for dodge AC from different sources.

Differentiate AC types: Full AC includes all bonuses. Touch AC excludes armor and shields. Flat-footed AC excludes dodge bonuses.

Be aware of AC vulnerabilities: Losing initiative can lead to being flat-footed, losing all dodge bonuses. Touch attacks bypass armor and shields.

Consider concealment: Spells like Blur, Mirror Image, and Displacement can provide concealment to avoid being hit.

Flanking: When two creatures attack the same opponent, the opponent is flanked, granting attackers +2 to attack rolls. This increases to +4 with the Outflank teamwork feat. Ranged characters cannot flank.

Sneak Attacks: Characters dealing sneak damage can apply bonus damage to enemies who are flanked, flat-footed, or denied their Dexterity bonus to AC. Ranged attackers can only deal additional sneak damage to enemies flanked by two or more melee attackers.

Charging: If you are 10' or more away with a clear path and no difficult terrain, you can make a charge attack as a full-round action, gaining a single attack at full AB +2.

Calculate damage: Damage is modified by the wielder's Strength bonus (or Dexterity with appropriate feats).