і / о

This alternation is a hallmark of Ukrainian phonology. Historically, the loss of short vowels (known as 'jers') caused the preceding о and е to lengthen and change into і in closed syllables.

Key Conditions:

• Closed Syllable: Ends in a consonant. Vowels о and е change to і. Examples: стіл (table), вечір (evening), осінь (autumn).
• Open Syllable: Ends in a vowel. The vowel remains о or е. Examples: сто-ла, ве-чо-ра, о-се-ні.

Common Contexts:
1. Noun Declension: Seen in Nominative Subject (closed) vs. oblique cases like Genitive, Dative, or Instrumental (open, e.g., стіл → стола, столу, столом).
2. Genitive Plural: A word like шко-ла (open) loses its ending in the Genitive plural, closing the syllable: шкіл.
3. Past Tense Verbs: The masculine past tense has a zero ending (closed syllable, e.g., ніс, міг), while other forms add vowels (open syllables, e.g., несла, могли).
4. Word Derivation (Словотворення): Alternation happens when forming new words (e.g., робота → робітник, гора → гірський).

Important Exceptions:

• Pleophony: Vowel groups -оро-, -оло-, and -ере- usually do not alternate in masculine nouns (e.g., closed syllables like мороз and берег do not change to моріз or беріг). However, they frequently alternate in the genitive plural of feminine and neuter nouns (e.g., голова → голів, дорога → доріг). Four common masculine exceptions do alternate: поріг (порога), сморід (смороду), моріг (морога), and оборіг (оборога).
• Fugitive Vowels (Випадні голосні): If о or е drops out entirely when declined, it does not change to і (e.g., сон → сну).
• Inserted Vowels (Вставні голосні): Vowels that are inserted to break up consonant clusters in the Genitive plural do not change to і (e.g., вікно → вікон, казка → казок).
• Consonant Clusters: Vowels in groups like -ов-, -ор-, -ер- usually do not alternate when they are between consonants (e.g., вовк, смерть).
• Specific Suffixes: Suffixes like -очк-, -ечк-, -енк- do not alternate even when the syllable becomes closed (e.g., Genitive plural казочок, донечок).
• Neuter Nouns in -ення: In the Genitive plural, they lose their final vowel (closing the syllable), but the е does not change to і (e.g., значення → значень).
• Loanwords: Modern borrowings (директор, балкон) are stable, but older, integrated loanwords do alternate (колір → кольору, папір → паперу).
• Compound Words (Складні слова): Compound words denoting professions or mechanisms ending in -вод, -воз, -роб, -нос do not alternate (e.g., хлібороб → хлібороба). However, compound words denoting inanimate systems or objects do alternate (e.g., водопровід → водопроводу).