The D&D core mechanic is:

1. roll d20
2. add or subtract modifiers

The modifiers are really the core, the d20 is just the randomizer!

Modifiers come in several “flavors” (but they are really all the same)

• Attribute bonuses
• Skills and proficiencies
• Attack Bonuses
• Saving Throws

• Initiative is just a d20 modifier. Highest roll goes first, and sticks for whole encounter
• Take turns - Action + Move (+ bonus actions)
  • Attack, Cast, Dash, Disengage, Dodge, Help, Hide, Use Item, Search, Ready
• Reaction (1 per round)
  • including Opportunity Attack (voided by Disengage)
• Attack (d20+ATK Mod) >= Armor Class == hit
  • roll Damage
  • NO Saving Throws on Attacks, attack go against AC
• SOME spells are automatic (no ATK roll) but do offer Saving Throw