The monophthongization of diphthongs restructured the Proto-Slavic language with a strong influence on its morphophonology.
Proto-Balto-Slavic, the ancestor of Proto-Slavic, possessed three vocalic diphthongs: *ai, *au and *ei.
The fourth Proto-Indo-European vocalic diphthong, *eu, had already become *jau in Proto-Balto-Slavic.
The unrounding of older long *ū to Slavic *y had already taken place by the time of the monophthongization; the new *u filled the gap left by it.
George Y. Shevelov dates the monophthongization of diphthongs to the 5th – 7th century AD.