In short

Ukrainian roots are not spelled the same in every form of a word. Two very common vowel alternations reshape о and е:

• о/е → і in a closed syllable. When the last syllable of a word ends in a consonant (the bare nominative of many nouns), an underlying о or е surfaces as і. Add a case ending that opens the syllable again and it reverts to о or е: кіт but кота́, ніч but но́чі, ве́чір but ве́чора.
• Fleeting (випадні) о/е. Some о and е exist only to prop up a final consonant cluster; the moment an ending is attached they vanish completely (they do not turn into і): у́чень but у́чня, у́чні; ти́ждень but ти́жня; день but дня.

Recognising both patterns lets you decline nouns correctly and spell what you actually hear.

The rule

These are two different mechanisms that happen to touch the same two vowels. One swaps о/е with і; the other drops о/е. Do not confuse them.

1. о/е → і in closed syllables (закри́тий склад)

Historically, when a syllable containing о or е became closed — that is, its vowel was the last sound before a word-final consonant — that о or е changed to і. When a case ending adds a vowel, the syllable opens again and the original о/е comes back. So across a single paradigm the same root alternates: і in the closed form (usually the nominative singular, and for feminine III-declension nouns also the accusative and instrumental), о/е wherever an ending opens the syllable.

• о ↔ і: кіт – кота́, ніч – но́чі, ніс – но́са, ріг – ро́га, стіл – стола́.
• е ↔ і: ве́чір – ве́чора, о́сінь – о́сені, ка́мінь – ка́меня.

This alternation is lexical and historical — it affects many words but not all. Words such as моро́з and вовк simply keep their vowel (there is no *мріз). Because you cannot predict it from spelling alone, learn each word as a nominative + genitive pair: if the genitive is кота́, the nominative is кіт.

2. Fleeting (випадні) о/е

A second group of о and е exist only to make a word pronounceable when it ends in a consonant cluster — they are “inserted” vowels. As soon as an ending is attached, the cluster is no longer word-final and the vowel drops out entirely. It does not become і; it disappears.

These fleeting vowels cluster in a few suffixes:

• -ень: у́чень → у́чня → у́чні; ти́ждень → ти́жня → ти́жні; день → дня → дні.
• -ок: буди́нок → буди́нку; шмато́к → шматка́; пісо́к → піску́.
• -ець: хло́пець → хло́пця → хло́пці.
• other stems: ві́тер → ві́тру; сон → сну; вого́нь → вогню́.

The same drop runs in reverse in the genitive plural of many feminine and neuter nouns, where a vowel is inserted to break up a newly word-final cluster: вікно́ → ві́кон, сестра́ → сесте́р. And in a genitive plural whose final syllable ends up closed, the first rule kicks in too and the root о surfaces as і: нога́ → ніг.

Forms & patterns

Closed-syllable о/е → і. In these paradigms і shows up only where the syllable is closed; everywhere an ending opens it, о returns.

кіт (masc., animate) — і only in the nominative singular:

ніч (fem., III declension) — і in the nominative, the identical accusative, and the instrumental, where the doubled consonant of ні́ччю (ні́ч-чю) keeps the syllable closed:

Fleeting о/е. The vowel survives only in the bare nominative (and, for inanimate nouns, the identical accusative); every ending erases it.

у́чень (masc., animate) — е appears only in the nominative singular:

ти́ждень (masc., inanimate) — е appears in the nominative and the identical accusative:

Examples

Common mistakes

• кіта́ -> кота́ — the syllable opens in the genitive, so і reverts to о; і stays only in the closed nominative кіт.
• ні́чі -> но́чі — the genitive syllable но-чі is open, so use о; і survives only in ніч, ніч (acc.) and ні́ччю.
• у́ченя -> у́чня — the fleeting е drops the instant any ending is added; it lives only in the bare nominative у́чень.
• ти́ждені -> ти́жні — the plural drops the fleeting е (ти́ждень → ти́жні); never keep it.
• де́ня -> дня — день has a fleeting е that disappears in every oblique form.
• мріз -> моро́з — not every closed-syllable о becomes і; this alternation is lexical, and моро́з simply keeps its о.

Tips

• To find the “hidden” vowel, decline the word: the oblique cases reveal о or е, while the closed nominative shows і. If the genitive is кота́, the nominative must be кіт.
• Because both alternations are triggered by the phonology of endings, expect them throughout the paradigm — every case, singular and plural — not just in one form.
• Fleeting vowels almost always sit right before the final consonant of the suffixes -ень, -ок, -ець (and in ві́тер, сон, вого́нь). Suspect a drop whenever you see those.
• Memorise fleeting-vowel nouns as a nominative + genitive pair (у́чень / у́чня, ти́ждень / ти́жня): the genitive tells you at once that the vowel disappears.
• Watch the genitive plural, where both patterns meet: the inserted vowel is о or е, never і (вікно́ → ві́кон, сестра́ → сесте́р, not *ві́кін), while a root о/е can still become і in the now-closed syllable (село́ → сіл, нога́ → ніг).
• Spelling follows sound in Ukrainian: write exactly what you say — кіт (not *кот), у́чні (not *у́чені).