Two things change, and neither one is really about the language sounding different. Ukrainian stress is free: it can land on any syllable, it is not predictable from spelling, and it can move to a different syllable when the same word changes case or number. Unstressed vowels also keep most of their color instead of fading toward a neutral sound, which is what English and Russian both do in their own ways. A topline built on a fixed stress pattern, or on the assumption that an off-beat syllable will get swallowed, runs into both problems the moment the lyric is Ukrainian.
Why is Ukrainian stress called free?
French stresses the last syllable of a phrase. Czech and Finnish stress the first syllable of a word. A composer working in either language can build a rule of thumb once and reuse it. Ukrainian offers no such shortcut: stress is phonemic and not predictable from the written word, and it can be the only thing separating two different words. Плачу́, stressed on the second syllable, means I pay. Пла́чу, stressed on the first, means I cry. Nothing in the spelling marks the difference, only the stress does, and a composer guessing wrong picks the wrong word as much as the wrong note.
Does the stress even stay on the same syllable?
Not always, and this is the part that catches people who learned a word's stress once and assumed it was fixed. Some nouns keep their stress on the stem in both singular and plural, the way теа́тр and теа́три both stress the first syllable. Others move it: жі́нка, woman, stresses the first syllable, while жінки́, women, stresses the last. A melody built around жі́нка, with the strong beat landing on жін-, does not automatically survive a late lyric edit that changes the word to its plural. The strong beat is now sitting on the wrong half of a different word, and nothing in the notation shows the mismatch until someone sings it.
Why doesn't an unstressed syllable just get swallowed?
In English, an unstressed vowel commonly fades toward a neutral schwa, which is why a composer can drop a quick passing tone on a weak syllable and expect the ear to gloss over it. Russian goes further in its own direction: unstressed о reduces toward a, an effect called akanye, so молоко is pronounced closer to malako than to its spelling. Ukrainian does neither. Unstressed /a/ centralizes slightly toward [ɐ] and unstressed /ɔ/ tightens toward [o], but both stay identifiable vowels rather than collapsing into a shared neutral sound, and the language has no equivalent of akanye at all. A fast note placed on an unstressed Ukrainian syllable is still heard as a clear, specific vowel, not as filler, so a careless melodic choice there is more exposed than the same choice would be in English.
Where this actually shows up in a session
Melismas are the first place it bites. Two or three notes stacked on one syllable ask a singer to sustain a vowel, and that sits naturally on a stressed syllable with an open vowel and awkwardly on an unstressed one, because the listener still expects that syllable to pass quickly. A habit carried over from writing English toplines, where the melisma goes wherever the phrase needs it, can land on the wrong syllable in Ukrainian and read as the wrong word getting the emphasis. The second place is any late rewrite. If a lyricist changes a word's case, number or tense after the topline is set, the stress can move even though the note assignment does not, and the fix is to re-mark the stress on the new word rather than assume the old note placement still lines up.
Why this matters for a contest entry specifically
Eurovision has allowed free choice of language since 1999, after two decades under a rule that tied each country to one of its own official languages, and Ukraine's entries have used Ukrainian, English, and both in the same song in different years. An entry that exists in a Ukrainian version and an English version is not one topline with translated words underneath. It is two separate stress-and-vowel passes, because a melody fitted to one language's stress pattern was never fitted to the other's, and treating the second version as a straight lyric swap is where the mismatch gets recorded.
A concrete check before locking the topline
Mark stress on the lyric sheet syllable by syllable before assigning a single note, using a stress mark over the vowel rather than trusting memory. Do it again after every rewrite that touches word endings, since that is exactly where Ukrainian stress moves. Treat this as mandatory whenever a lyric passes through a second writer or a translator, because a stress-marking mistake made once at that stage is the most common reason a finished vocal sounds like it emphasizes the wrong word without anyone being able to say why.
Frequently asked
Can you translate an English lyric into Ukrainian and keep the same melody? Only if you recheck every syllable afterward. A word for word translation rarely keeps the stress in the same place.
Does free stress make Ukrainian harder to set to music than English? Different, not harder. English hides a misplaced note through vowel reduction. Ukrainian does not, but checked correctly, the words rarely fight the meter.
Is there a rule of thumb for where Ukrainian stress falls? No reliable one. It has to be checked per word, and checked again whenever the grammatical form changes.
If a lyric is moving between languages
Mark stress on every syllable before touching note placement, in either direction, then check it again after the last rewrite. Full credit list at /credits, background and contact at /about.