I want to know the answer to the following question:
Why aren't some sound clusters used in modern Czech?
All the background I'm provided with is that "A phonological phenomenon, Havlik's law (which began in Proto-Slavic and took various forms in other Slavic languages), appeared in Old Czech; counting backwards from the end of a clause, every odd-numbered yer was vocalized as a vowel, while the other yers disappeared.". Is that enough to answer the question?
Answer: no