This is the 8927th most frequent Turkish word.
sınıfın
"Of the class" or "the class's" (possessive form).
In this sentence, 'sınıfın' is used to indicate possession ('the class's') of the door.
Sınıfın kapısı açık.
The class's door is open.
Here, 'sınıfın' denotes that the window belongs to the classroom ('the window of the classroom').
Sınıfın penceresi çok geniş.
The window of the classroom is very wide.
In this context, 'sınıfın' is specifying the students who belong to or are part of the class ('the students of the class').