Vowel harmony:
A word may not have both vowel groups; back and front!
УУ , ҮҮ are used in questions that have no question word.
After a word ending with long vowel or diphthong: ЮУ, ЮҮ
Энэ дэвтэр ...., цаас ...?
Quelle: Сайн байна уу? L. Tserentschunt, S. Luethy. Ulaanbaatar 2000. S. 17