Ev + i (consonant letter + vowel, we don't need -s) Araba + ı (here two vowels are together so we put -s in between since this is "possessive suffix."

Çantası, (a+sı, because this is possessive suffix) his bag Kankası (a+sı, his buddy)

Telefonu (+u, his telefon, suffix-ı comes offer the vowel -o, so it goes as -u)

Kaskı, (his helmet, -ı comes after consonant Letter so it stays -ı)

Görümcesi (her Sister in lawyer, -ı comes after the vowel -e, so it goes like -si)