1. Could you open the window, please?
2. He opened the window.
3. Now there is an open window. [This is correct.]
Is the second sentence fine?
The word 'open' is both an adjective and a verb.
So as a verb, the forms are 'open' , 'opened' and 'opening'.
You use an 'opener' to open canned food.
You will find an 'opening' in a water tank; If there is an opening, the water will leak.