Questions & Answers about Onuncu soru zordu.
This is because of vowel harmony, a very important feature of Turkish.
The ordinal suffix changes shape to match the vowels in the word it attaches to. After on, the correct form is -uncu, giving onuncu.
You do not need to memorize a separate rule just for ten; it is part of the normal vowel-harmony pattern seen in ordinal numbers:
- altı → altıncı
- dokuz → dokuzuncu
- on → onuncu