Understanding Existing Rules

In addition there is a section for House Rules. RAW: Rules as Written; RAI: Rules as Intended; RAP: Rules as Played

--Damage, when halved, is rounded down; e.g. 7 pts of fire damage, half is 3 pts. There’s one more general rule you need to know at the outset. Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater.

--Resistance to damage can only be halved once. Dodging or avoiding for half damage is not the same as resistant thus if a character dodges for half damage, say 15 is halved to 7, then is resistant too they take 3 pts of actual damage. Resistance and then vulnerability are applied after all other modifiers to damage.

--A tie goes to the status quo or defender.

--Invisibility Spell effects:  "A creature you touch becomes invisible... Anything the target is wearing or carrying is invisible as long as it is on the target's person." Something later picked up, put on, carried, or worn is NOT invisible, a creature carried by the target does not turn invisible. If an already invisible item is used to cover or wrap the visible item then it becomes invisible. --It does NOT grant Advantage on Stealth check, rather Attack or WIS (Perception-Sight) checks versus the invisible target would be at Disadvantage. --Casting a spell via a familiar is casting a spell which like attacking cancels the Invisibility.

--Secret doors or traps:  "When a character searches for hidden objects such as a secret door or trap, typically make a WIS (Perception) check." Perception is for observation, Investigation is for deduction. Traps can be noticed with Passive Perception (usually DC 15). If the PC is looking, how they are looking? If it's observational, then Perception. If it's deductive, use Investigation. When they are searching for items, again, either skill can be appropriate. Modules are always written to use perception to notice traps. Passive with a higher DC and active with a lower. To notice a magical trap use INT (Arcana), to disarm could be either INT (Arcana) or Dispell Magic.

--Complex traps vs Simple traps: More difficult to disable than simple ones, a complex trap requires more than a single check. After a complex trap activates, it remains dangerous round after round until the characters avoid it or disable it. Only one successful ability check is required to disarm a simple trap; DEX (thieves’ tools) apply to any trap that has a mechanical element, STR is the method for thwarting traps operating through the use of brute force, and INT (Arcana) enables one to figure out how a magic trap functions and how to negate its effect.

--Haunted trap:  Thieves’ tools and Dispel Magic do not affect haunted traps, however, the Channel Divinity class feature and the Remove Curse spell can disarm a haunted trap.

--Spell Description: Most spell descriptions are very specific for example Magic Missile states "hits a creature of your choice", while Fire Bolt states "hurl a mote of fire at a creature or object within range." The rules do make the distinction between "animated" creatures and "inanimate" objects. Constructs are animated and considered creatures.

--Spell Identification: One can use Reaction to identify a spell in the moment, or use an Action on their next turn. It takes INT (Arcana) with DC equals 15 + spell's level. If cast as a class spell, a member of that class can make the check with Advantage.

--Area of Effect (five shapes):  A cone, cube, cylinder, line, or sphere with a specified point of origin. A spell's effect expands in straight lines from the point of origin. [As D&D 5e is not a highly detailed tactical game, and without extensive homebrew or house rules, a straight line follows the square or grid system. This does not allow for alternate shapes or redefinition of a square to a diamond.]

--Concentration: As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly. There are only a few cases where casting a new spell will interrupt concentration of a previously cast spell.  (1) The new spell is another concentration spell. As soon as you start casting it - the concentration on the previous spell will break. (2) Casting a spell to hold it for later in the round. Holding a spell requires concentrating on it and will therefore break concentration of another spell already up. (3) Casting a spell with a casting time greater than 1 action.