Could you please answer?
Which of the sentences are right. On the door there is a curtain. or At the door there is a curtain.
And
On the window there is a curtain. or At the window there is a curtain.

thank you

Question

Could you please answer?
Which of the sentences are right. On the door there is a curtain. or At the door there is a curtain.
 And
On the window there is a curtain. or At the window there is a curtain.

thank you

Answer

On the door there is a curtain.
On the window there is a curtain.

however a more natural way of saying would be -> The door has a curtain. The window has a curtain.

Answer

If the curtain is covering the surface of the door, "on".