From Wiktionary, the free dictionary
-sunuz
- Form of -siniz after the vowels O / U.
-sunuz
- Suffix of verb "to be" for subject "you" (formal or plural).
- Yorgunsunuz. ― You are tired.
- It's used when the word's last vowel is one of "o" or "u".
- It could be "-sünüz", "-siniz", "-sınız" according to the last vowel of the word.
- While talking about you (singular) or you (informal), it becomes "-sun"