-и / -ију

In standard Serbian, the default ending for the genitive plural across masculine, neuter, and feminine nouns ending in -а (the first two declensions) is a long -ā (as in кућа, градова, села). However, the language also employs alternative or dedicated patterns with -и and -ију, alongside the ending -у. The ending -у is an organic dual preservation in paired body parts like руку and ногу, whereas in слугу it spread as an analogical development. Understanding these alternatives is essential to becoming confident with plural noun inflection.

Crucial Pronunciation Note:
Just like the default genitive plural -ā, the ending -и carries mandatory post-accentual length (-ӣ). In feminine nouns ending in a consonant, the final length on -ӣ is additionally accompanied by an accent shift: monosyllabic stems with a falling accent in the singular shift to a rising accent in the genitive plural. Long stems shift to long-rising (G.sg. ре̑чи vs. G.pl. ре́чӣ, G.sg. ства̑ри vs. G.pl. ства́рӣ), while short-stem alternating nouns shift to short-rising (G.sg. но̏ћи vs. G.pl. но̀ћӣ). Similarly, the suppletive plural noun људи changes its pitch alongside gaining final length: nominative plural љу̑ди (long-falling) contrasts with genitive plural љу́дӣ (long-rising on the stem plus final post-accentual length).

1. The Ending -и:

• Feminine consonant nouns (III declension): The ending -и is not an irregular alternative here, but rather the regular, systemic ending for this entire noun class: ствар → ствари, ноћ → ноћи, реч → речи, мисао → мисли.
• Masculine units of time and measurement: Specific masculine nouns take -и instead of -а when referring to units of measurement: сат → сати (hours), месец → месеци (calendar months; whereas the celestial body 'moons' takes regular месеца), and степен → степени (degrees of temperature or angles; whereas stages, academic degrees, or grammatical degrees take regular степена).
• Irregular masculine plurals: The suppletive plural noun људи (from човек) regularly takes the ending -и (љу́дӣ).
• Consonant clusters in nouns ending in -a: When a noun stem ends in a consonant cluster where inserting a fleeting a (непостојано а) is awkward, phonologically blocked, or disfavored, the ending -и is frequently used. This includes native formations with sonorant + б clusters like -лб- and -рб- (молба → молби, борба → борби, жалба → жалби) as well as loanwords and stems with clusters like -пт- or -мб- (лопта → лопти, бомба → бомби, лампа → лампи). Note that clusters ending in -ст, -зд, -шт, and -шћ take standard -а without changes (e.g., врста → врста, башта → башта).
• Normative preference with doublets: When doublets exist between a fleeting a form and an -и form, standard Serbian grammars maintain a strong normative literary preference for the forms with fleeting a: примедаба is preferred over примедби, битака over битки, and погрешака over погрешки, though the -и variants are widely heard in colloquial speech.

2. The Ending -ију (Dual Remnant):
This ending is fossilized and restricted to a small lexical set. You cannot freely attach -ију to regular nouns:

• Body parts: око → очију (eyes), ухо / уво → ушију (ears), прст → прстију (alongside regular прста), нокат → ноктију (alongside ноката), кост → костију (alongside кости).
• Non-body nouns: гост → гостију (alongside гости), кокош → кокошију (alongside кокоши).

Important Note on Counting vs. Other Syntactic Contexts:
Like all genitive plurals, forms in -и and -ију appear wherever the genitive plural case is syntactically required—such as after prepositions (испред очију, после неколико сати), in possessive expressions (боја очију), or with negated verbs. When counting, remember that 2, 3, and 4 govern the paucal form, whereas numbers 5 and above and indefinite quantifiers require the genitive plural: три сата vs. пет сати; два госта vs. много гостију.