What is the difference between "the open door" and "the opened door"? Are they the same in meaning? Please explain it. Thanks a lot.
Open is an adjective.
Opened is a past participle used as a modifier.
The difference in meaning is that open is in contrast to not open. The door could have been open for ten years or a century. It could always be open.
Opened expands to The "having been opened" door...
This refers to the action of the door having been opened. The door used to be shut, now it is open. It has been opened. It has not always been open. It could have been opened ten years ago or two seconds ago, but at some point it was closed.
I think the difference between the open door and opened door is one of time. The open door refers to one that is open and has always been open, the opened door refers to one that has only recently been opened.
In practice, they are exactly the same thing. But open door is the common expression. Nobody says opened door.
In terms of English rules and all, see other answers. In terms of natural English, see this answer.