Questions & Answers about Onun arabası büyük ve kapısı açık.
Why do both arabası and kapısı end with -sı?
The base words are arabası (car) and kapı (door), which both end in a vowel. The 3rd person possessive suffix (meaning "his", "her", or "its") is a single vowel: -ı, -i, -u, or -ü. To stop two vowels from crashing together, we add the buffer letter s. So, araba + s + ı becomes arabası (his car), and kapı + s + ı becomes kapısı (its door).
I thought the buffer letter between two vowels was usually 'y'. Why use 's' here?
You are right that y is the most common buffer letter in Turkish. However, the 3rd person possessive suffix is a special exception! It is the only suffix that uses s as a buffer letter. If you see -sı, -si, -su, or -sü added to a noun, it almost always means "his", "her", or "its".
The translation says "its door", but I only see "onun" (his) at the beginning. Where does "its" come from?
The "its" is built right into the word kapısı! Because of the -sı suffix, we know the door belongs to a third person or thing. In Turkish, if the context is obvious, you do not need to repeat the pronoun. Since we are already talking about the car, kapısı naturally means "its door" without needing to say "onun" again.
Why isn't there a word for "is" after büyük (big) and açık (open)?
In Turkish, you do not need a separate word for "is" or "are" in simple present tense sentences like this one. You just place the subject and the adjective side by side. Onun arabası büyük literally translates to "His car big", and kapısı açık means "its door open". The "is" is simply understood from the context.