Sub-event means that if the previous event is true, so the next events that is "inside" of another event will run.
Doens't exist an "open door" command. What Rabenmutter meant is that you need to create an sprite and load an image of a door.
Then you an event like this:
if character is overlapping the sprite "door"
than go to layout "room"
So you need to create another layout with your map "room" before doing this
Edit: forget it haha
"on start of layout" -> spawn the object.
There is spawn to another object and to a positon where you need to input the coordinates