With some verbs (not all) the past participles can be used as adjectives to describe the state of something or someone:
I am bored
I am confused
The door is closed
However, there are some exceptions, which are simply exceptions based on what people say. For example, we use the past participle for "closed" but use the base form (of the verb) for "open":
The door is open.
Odd...... but an exception.
If we say:
The door is opened
The psychology raises questions, as it sounds like passive voice... "The door is opened by the concierge" and the message can be confusing to the native speaker who has always heard and said:
The door is open
Sometimes there is no explanation other than these things can be a simple exception as to the way we talk. In these cases, memorization is the only rule.
Good luck!