Collective and suppletive plurals

Three of the most frequent words for people have no ordinary plural ending at all. Instead each has a completely different word for 'many': brat → braća (brothers), dete → deca (children), čovek → ljudi (people). A fourth, gospodin → gospoda (gentlemen), works exactly like braća. The words themselves are easy to learn. The real difficulty is how the words around them agree.

braća and deca: grammatically one feminine noun

Braća and deca mean several people, but in form they are a single feminine noun ending in -a, like žena. They decline like žena in the singular: braća, braće, braći, braću, braćom — never with plural endings like -ama. These are called collective nouns: one word for a whole group.

Ljudi is different: it is a real masculine plural, declined like prijatelji — only the stem is borrowed from another word. So ljudi takes ordinary masculine plural agreement: ovi ljudi, s ljudima. The one surprise is the genitive plural, which is ljudi again rather than ending in -a like prijatelja: mnogo ljudi.

Agreement: modifiers are feminine singular, the verb is plural

Here is the heart of the lesson. With braća and deca, a word that DESCRIBES the noun (a possessive, a demonstrative, an adjective) takes the feminine singular form, in whatever case the noun is in: moja braća, moje braće, s mojom braćom, mojoj deci, svoju malu decu. But the VERB is plural: deca spavaju, braća žive, braća su stigla. In the past tense the participle and any predicate adjective end in -a with su: deca su zaspala, deca su bila umorna.

Ljudi follows none of this: it takes masculine plural everywhere, like any masculine plural noun — ovi dobri ljudi, ljudi su došli.

Two classic slips. First, do not make the modifier plural: it is moja braća and s mojom braćom, never with moji or mojim. Second, do not make the verb singular: it is deca su došla and deca spavaju, never with je or spava. Describing word: feminine singular. Verb: plural.

The genitive: counting and 'how many'

After mnogo, koliko, nemam and similar, braća and deca take their feminine singular genitive (braće, dece), while ljudi takes the genitive plural, which is also ljudi: Koliko imaš braće? Nemam dece. Bilo je mnogo ljudi. With a describing word, the same split shows: mnogo male dece but mnogo dobrih ljudi.

• Two to four brothers are counted with the singular word: dva brata, tri brata.
• Children are counted with collective numerals: dvoje dece, troje dece.
• People: dva čoveka, tri čoveka, but pet ljudi, mnogo ljudi.

Addressing a group uses its own form: Deco! (Children!), Braćo! (Brothers!), Ljudi! (People!), and the fixed Dame i gospodo! (Ladies and gentlemen!).