Questions & Answers about Komşum kapıyı çalıyor.
-m is the 1st person singular possessive suffix, meaning my.
- komşu = neighbor
- komşum = my neighbor
So the sentence literally starts with My neighbor...
Because kapı is the direct object (the thing being knocked), and Turkish marks definite direct objects with the accusative case: -(y)I.
- kapı = door
- kapıyı = the door (as a specific/definite object)
The -y- is a buffer consonant inserted because kapı ends in a vowel, so you don’t get two vowels in a row: kapı + ı → kapıyı.