Gabriel Moon, You are right. Both can be used. "Open" is much more common as an adjective than "opened" when referring to doors, windows, etc. Could you explain to me why only the word "open" should be used instead "opened"? The word "open" first came into the English language as an adjective. So an expression like "The window is open” was in use before the verb "to open" even existed. The past participle "opened” means "made open". It is normally used as a descriptive adjective before a noun. Example: The beer was foaming from the mouths of the opened bottles. I would say that "I keep the windows open." is the without question the normal way to express the thought, but "I keep the windows opened." is gramatically correct. -------------------- ...by contrast, not the word "close" but the word "closed", the past participle of the verb "close" is used in other sentence? The word "close" entered into the English language as a verb along with it the past participle "closed". "Closed" has the opposite meaning of "open" or "opened' when referring to windows, doors, etc. "The windows is closed." is the normal way to express the idea. The adjective "close" meaning “closed” came into existence later. It is very rare and is not used in Modern English when referring to windows, doors, etc.

In common speech both can be used without issue.

Because the word "open" in the first sentence is the adjective. So you can see the open door, the open window. So keep the open windows or keep the windows be open. Here you use the verb "keep" in this case. And of course you may do not use the construction "keep" with the "window, which was or is opened".

In the first sentence, you describe HOW. "Open" actually works as an adverb here. In the second sentence, you describe WHAT HAPPENED to the windows. Somebody opened them (but you don't say who). "Keep the windows opened" sounds unnatural. Your basic problem is that you're assuming English can be treated like a lego set. It's a language, so you need to accept it's a little more complex than basic rules.