on and after

Behavior attaches to its owner as an on or after clause inside the owner's create block (or inside a trait). The two words divide one moment: on runs before the action commits and may stop it; after runs once it has happened and reacts.

create the hive box
  aka hive, box
  scenery, openable
  in the Apiary
  states: humming, quiet

  A white wooden hive box, lid held down by a brick.

  on the player opening
    refuse when the hive box is humming: bees-disturbed
    change the hive box to quiet
  end on

  after the player opening
    phrase honey-smell
  end after

The division is enforced: refuse, refuse when, and must are parse errors inside an after body (parse.react-refusal). If a reaction wants to stop something, it is not a reaction; move it to on. Every clause closes with an end line matching its opener.

The head names who acts

A clause head is actor, then verb: on the player opening. The block it sits in is what is acted on — the hive box above — so the head has one job, naming who is doing it.

create Marguerite
  a person
  starts in the Counting House

  The senior clerk, and she knows it.

create the ledger
  in the Counting House

  A ledger, spine cracked.

  on the player taking
    refuse ledger-not-yours
  end on

  after Marguerite taking
    phrase she-checks-the-column
  end after

The two clauses sit on the same object and fire for different people. Write the owner's name where you would once have written it.

the player is a role, not a character

the player means whoever is the player character when the clause fires, so a clause written with it follows the role if the story hands it to someone else. To pin behavior to one particular character instead, write that character's name.

A character's own block needs no name

Inside a person's create block the subject is already settled, so the head drops the actor — the same way wears the boots needs no subject.

create Marguerite
  a person
  starts in the Counting House

  The senior clerk, and she knows it.

  on going
    refuse she-will-not-leave-the-ledger
  end on

That bare head is her own going. Outside a person's block it is an error (analysis.head-bare-outside-actor) — a hive box does not open things, so on opening there has no one to mean.