Rhythmic law
rytmický zákon
Native Slovak generally avoids two successive long syllables.
Hear and produce the shortening that prevents two successive long syllables.
In native Slovak words, two long syllables usually do not follow one another. A long vowel or diphthong counts as a long syllable nucleus, so the next syllable is often shortened.
This is why speakers say krásny rather than *krásný, bývam rather than *bývám, and múdry rather than *múdrý.
Loans, some morphological environments, and compounds can be exceptions. Treat the rhythmic law as a strong native-word tendency, not a rule for generating every form.
Sound contrasts
krásny / krásný standard Slovak / two long syllables avoided
bývam / bývám standard Slovak / second vowel shortened
múdry / múdrý standard Slovak / second vowel shortened
How it is formed
Hold the first long vowel or diphthong, then keep the following syllable short and light instead of stretching both.
Words and phrase
krásnybývammúdrymlieko
Bývam v krásnom dome. I live in a beautiful house.
Reference
Jazykovedný ústav Ľudovíta Štúra SAV ↗Full attribution on References.