This lecture discusses the changes in the vowel system caused by the development of hard/soft correlation, weakening of the backness distinction in vowels and loss of the law of intrasyllabic vowel harmony: loss of the front phonemes /ä/ и /ü/, which became allophones of the phonemes /а/ и /u/; the history of correlations between the phonemes /i/ and /у/, and between /е/ and /о/. Besides, the lecture deals with the shifting of /е/ into /о/, and the loss of the phoneme 'љ' and the closed /ô/.