Core Resolution: Checks and DCs
All PF2e checks use d20 + modifier vs DC. Bonuses of the same type don't stack. Flat checks are pure-chance d20 rolls with no modifiers.
Every meaningful action in PF2e resolves the same way: roll a d20, add your modifier, and compare to a Difficulty Class (DC). This single mechanic underpins attack rolls, saving throws, skill checks, and flat checks.
The Formula: d20 + ability modifier + proficiency bonus + item bonus + circumstance bonus + status bonus. Bonuses of the same type do not stack — only the highest applies. Penalties of the same type also take only the worst value, except untyped penalties which always stack.
Difficulty Classes: The GM sets a DC for tasks. Simple DCs range from 10 (trivially easy) to 50 (nearly impossible). Level-based DCs start at 14 for level 0 tasks and increase by roughly 1-2 per level. Creature DCs use 10 + the creature's relevant modifier. Opposed checks use the opponent's DC (10 + their modifier) rather than both sides rolling.
Flat Checks: Some situations call for a flat check — a d20 roll with no modifiers. These represent pure chance: targeting a concealed creature (DC 5), targeting a hidden creature (DC 11), or shaking off persistent damage (DC 15). No bonuses or penalties apply unless a rule specifically says so.