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Questions & Answers about Odanın penceresi açık.
Why does oda (room) take the suffix -nın instead of just -ın?
The genitive suffix (meaning 's or of) is normally -ın, -in, -un, or -ün. Because oda ends in a vowel, we need a buffer consonant to prevent two vowels from touching. For the genitive case, this buffer letter is always n, so it becomes -nın.
Why does pencere (window) take the suffix -si instead of -ni?
The word pencere (window) is the possessed item here. The 3rd-person singular possessive suffix is normally -ı, -i, -u, or -ü. When the word ends in a vowel, it needs a buffer letter. For the 3rd-person possessive, that buffer is always s, making it -si. Notice how the genitive buffer is n (odanın) but the possessive buffer is s (penceresi)!
Could I just say oda penceresi without the -nın on oda?
You could, but it changes the meaning. Odanın penceresi uses the genitive suffix to form a definite noun compound, meaning the window of a specific room. If you drop the genitive suffix and say oda penceresi, it becomes an indefinite compound meaning a room window (the type of window, rather than a specific room's window).
Where is the word for is in this sentence?
In Turkish, there is no separate word for is or are in simple present sentences like this. For third-person subjects (like the window), the is is simply understood from the context. So açık functions as is open without needing an extra verb.