Global values - values that vary
Start is a room. "You are at the start of the game. [if the era is the past] In the past[otherwise] In the present[end if]."
The time machine is a thing in the start.
Instead of examining the time machine:
say "You feel your mind drifting...";
now the era is the past.
Openable things that change when you open them
A bottle is thing in The Beach. "You see a weather-worn, smoothly rounded bottle."
A bottle is a closed openable container.
The description of A Bottle is "You look through the frosted glass surface, but you can only make out a vague shape. The bottle is sealed with a cork."
Understand the command "uncork" as "open".
Instead of opening a bottle:
say "With some difficulty you extract the cork from the neck of the bottle.";
now the description of a bottle is "Peering through the bottle neck you see a tiny model ship.";
Defining new verbs
Understand "turn down volume" or "turn down music" or "turn down the volume" or "turn down the music" as lowering the volume. Lowering the volume is an action applying to nothing.
Instead of lowering the volume, try setting the volume knob to 1.