к / ц, г / з, х / с перед -і

In Ukrainian, the velar consonants к, г, х undergo a specific alternation when they are followed by the inflectional ending -і. This occurs primarily during noun declension and results in soft (palatalized) sounds.

The Three Shifts:

• к → ц: рука (hand) → на руці (on the hand)
• г → з: нога (leg) → на нозі (on the leg)
• х → с: вухо (ear) → у вусі (in the ear)

Where it happens:
1. Feminine Nouns: In the Dative (e.g., допомогти жінці - to help a woman, подарунок Ользі - a gift for Olha) and Locative singular. The Locative case is often triggered by prepositions like в/у (in), на (on), по (along/around), and при (at/by/during), making phrases like по дорозі (along the road) or по річці (along the river) very common.
2. Neuter Nouns: In the Locative singular, but only for those that take the -і ending (e.g., вухо → у вусі). Neuter nouns that take the -у ending do not alternate (e.g., ліжко → у ліжку).
3. Masculine Nouns: While many masculine nouns use the ending -у, the alternation is mandatory for those that take -і in the Locative, such as бік (side) → на боці or берег (bank) → на березі. A great minimal pair to illustrate this is the masculine банк (bank), which takes -у and has no alternation (у банку), versus the feminine банка (jar), which takes -і and alternates (у банці).

Consonant Clusters:
The rule still applies when the velar is part of a cluster. For example, in книжка (book), the -жк- cluster shifts to -жц-: у книжці (in the book). Similarly, дошка (board) → на дошці. Note that due to regressive assimilation, these clusters are pronounced as [зьц'] and [сьц'] respectively (e.g., книжці sounds like [книзьці], and дошці sounds like [досьці]).

Important Notes:
• This is an inflectional rule that strictly applies at the morpheme boundary (stem + ending). When 'і' is part of the root, the consonant does not change. This applies to both native words like кіт (cat) or гілка (branch), and loanwords like кіно (cinema) or гітара (guitar).
• This change does not happen before the letter и. For example, the plural книги (books) keeps the 'г' because the ending is -и, not -і.
• Locative Plural: The Locative plural endings are -ах/-ях, not -і. Therefore, the consonant shift does not occur in the plural (e.g., на руках - on the hands, у вухах - in the ears).
• Irregular Plural: While most masculine nouns take -и in the plural, the highly frequent word друг (friend) has an irregular plural ending in -і, which triggers the shift: друзі (friends).
• Plural adjectives ending in -і do not undergo this alternation. For example, великий (big) becomes великі in the plural, not велиці.