his dark materials

[دانیال]

why Lyra & will could not open another window between the world when there was already an open window?

[Hollyhocks]

Because in the end they had to close all the windows because the dust moving in between the worlds was destroying everything

[Joy]

Actually, one window was left open. The angel who talked to Will and Lyra about closing the windows told them they could leave one open. Will immediately thinks about the one where he first went through to Cittagaze and met Lyra. But Lyra realizes that the one window that has to be left open is the one from the world of the dead, otherwise the dead have to stay underground and never become dust themselves. Will and Lyra are faced with a choice--it's them and their love for each other or it is the dead being able to come into one of the worlds and be at peace. Even though it breaks both of their hearts and separates them forever, they choose to leave the window open so that the dead can continue to come through and add to the dust that is vital for the survival of all.