Numbers

Four numeral forms are used:
Cardinal numbers, један/jedan (one), два/dva (two), etc.
Ordinal numbers, први/prvi (first), други/drugi (second), etc.
Collective numerals, двоје/dvoje (a group of two mixed gender beings), троје/troje (a group of three mixed gender beings)
Number nouns, двојица/dvojica (a group of two male human beings), тројица/trojica (a group of three male human beings), etc. There is no special number form for counting females.

Cardinal numbers

Number one, jedan, behaves as a true adjective, agreeing with the noun in gender and case.
Jedan čovek. Jedna žena. Jedno dete.

All numbers ending in one (except compound number 11) follow the same rules.
Dvadeset jedan autobus je bio na stanici.
Trideset jedna zgrada je porušena.
Osamdeset jedno dete je bilo na izletu.

The negative form of the numeral one, nijedan, behaves exactly like jedan. The verb that follows it must be negated. When used with a preposition, the preposition must come between the prefix ni- and the number jedan.
Nijedan čovek nije došao.
Ni na jedno piće nije došao.

Numbers two, dva, three, tri, and four, četiri, behave differently from number one and from numerals five, six, seven, etc. The number two have two forms. One form is used for both masculine and neuter nouns and the other is for feminine nouns:
The ending -a is added to the stem of masculine and neuter nouns and adjectives following the number two, dva. This ending is only applied when the numbers are not declined:
Dva velika kamiona.
Dva nova telefona.

The ending -e is added to the stem of regular feminine nouns and adjectives following the number two, dva, while the ending -i is added to irregular feminine nouns (those ending in a consonant):
Dve velike kuće.
Dve lepe noći.

Ordinal numbers and their declension

All ordinal numbers function as definite adjectives, taking the gender, number and case of the noun they qualify.

m	f	n
prvi	prva	prvo
drugi	druga	drugo
treći	treća	treće
četvrti	četvrta	četvtro
peti	peta	peto
šesti	šesta	šesto
sedmi	sedma	sedmo
osmi	osma	osmo
deveti	deveta	deveto
deseti	deseta	deseto

Collective numerals

Collective numerals are used when referring to a group or collective of persons or animals of mixed gender and/or age.

Jedno, dvoje, troje, četvoro, petoro, šestoro, sedmoro, osmoro, devetoro, desetoro ...

Number nouns

Number nouns apply only to animate nouns of masculine gender. They refer to a group of male human beings, described by the number noun formed with the suffix -ica:

Dvojica, trojica, četvorica, petorica, šestorica, sedmorica, osmorica, devetorica, desetorica ...