This is the 14586th most frequent Turkish word.
penceresinin
"Of his/her/its window."
In this sentence, 'penceresinin' functions as a possessive form ('of the window') indicating ownership of the curtains by the window of the house.
Evin penceresinin perdesini değiştirdiler.
They changed the curtain of the house's window.
Here, 'penceresinin' again shows possession ('of the window'), but in this case, it refers to a hypothetical or situational window that belongs to the context of the tree/view mentioned.
Bahçedeki ağacın penceresinin tam önünde büyümeye başlaması tuhaf.
It is odd that the tree in the garden starts to grow right in front of the window.
In this sentence, 'penceresinin' serves as a possessive form ('of the window'), linking the glass specifically to the window of the library.