Monophthongization of diphthongs in Proto-Slavic
Okay kiddo, so you know how some words have more than one vowel sound in them? We call those words diphthongs. But a looooong time ago, in a language called Proto-Slavic, they decided to change those diphthongs into just one vowel sound. We call this "monophthongization."
So basically, instead of saying two vowel sounds one after the other, they started saying only one vowel sound for each diphthong. It's kind of like how you might say "apple" with two vowel sounds (ae) or you might say it with just one sound (a).
This happened in Proto-Slavic a very long time ago, but now lots of Slavic languages, like Russian and Czech, still use the monophthongized versions of those old diphthongs. It's just one of the ways that languages can change over time!
So basically, instead of saying two vowel sounds one after the other, they started saying only one vowel sound for each diphthong. It's kind of like how you might say "apple" with two vowel sounds (ae) or you might say it with just one sound (a).
This happened in Proto-Slavic a very long time ago, but now lots of Slavic languages, like Russian and Czech, still use the monophthongized versions of those old diphthongs. It's just one of the ways that languages can change over time!