There is little that can be said to introduce the cardinal numbers, so let's move on to the forms:
The numbers 0-10 are:
0. nula, 1. jedin (jedna, jedno), 2. dva (dvě), 3. tri, 4. četyri, 5. pet, 6. šest, 7. sedm, 8. osm, 9. devet, 10. deset.
The -teens (11-19) are formed by adding -nadset (pronounced -nacet) to the numbers 1-9:
11. jedinnadset, 12. dvanadset, 13. trinadset, 14. četyrinadset, 15. petnadset, 16. šestnadset, 17. sedmnadset, 18. osmnadset, 19. devetnadset.
The „-ties” (20-90) are formed by adding -deset to the numbers 2-9:
20. dvadeset, 30. trideset, 40. četyrideset, 50. petdeset, 60. šestdeset, 70. sedmdeset, 80. osmdeset, 90. devetdeset.
The hundreds (100-900) are formed by adding -sto to the numbers 2-9:
100. sto, 200. dvasto, 300. tristo, 400. četyristo, 500. petsto, 600. šeststo, 700. sedmsto, 800. osmsto, 900. devetsto
Alternatively, the hundreds can also be formed by inflecting the word sto, resulting in the following set:
100. sto, 200. dvěstě, 300. trista, 400. četyrista, 500. petsot, 600. šestsot, 700. sedmsot, 800. osmsot, 900. devetsot.
The words for „thousand”, „million” and „milliard” are: tyseč (1000), milion (106) and miliard (109). Just like in the case of the hundreds, these words may or may not be inflected as nouns. Although it is not customary in the Slavic languages to do this, in cases like pet-tyseć „5000” a hyphen may be added for clarity.
Combinations of these are always made from high to low: thousands – hundreds – tens – ones. Between the tens and the ones, the word i „and” may be inserted. For example: tri-tyseč četyristo petdeset (i) šest „3,456”.
Because the primary purpose of Interslavic is to be maximally understood by Slavic speakers, it deserves recommendation to write digits instead of number words.
For the average Westerner it would make most sense if the word jedin were followed by a noun in the singular, and all the remaining numerals by a noun in the plural: jedin dom, dva domy, pet domy, dvadeset domy, milion domy. However, in the Slavic languages things work differently. All numbers higher than 4 are followed by the genitive plural: pet domov (lit. „five of houses”). In the case of the numbers 2-4, most languages use the nominative plural in most cases, but some languages, notably Serbo-Croat and Russian, prefer the genitive singular instead. For reasons of clarity, however, it is recommended to use the nominative singular after 1, the nominative plural after 2-4 and the genitive plural after 5 and more: jedin dom, dva domy, četyri domy, pet domov.
The easiest solution for the declension of cardinal numbers is simply not to decline them at all. Dom s tri etažami „A house with three floors” is perfectly understandable, even though it may sound a little strange to native speakers. However, a better effect can be achieved by using the following declension patterns:
Except for the masculine nominative singular, the word jedin „one” is declined like an adjective *jedny: m. jedin, jednogo, f. jedna, jednoj, n. jedno, etc. In some cases, it can also be used in the plural: Jedni ljudi prědpočitajut lěto, drugi zimu „Some people prefer Summer, others Winter”. This is also the case for pluralia tantum: jedne dveri „one door”.
These numbers are declined in a somewhat unfamiliar way, because their patterns show remnants of the ancient dual. Only dva „two” has gender distinction, albeit only in the nominative/accusative.
The neuter of "2" is dva in Russian, Belarussian, Ukrainian, Polish, Cashubian and Serbocroat, dvě in Old Church Slavonic, Czech, Slovak, Upper and Lower Sorbian, Slovene, Macedonian, Bulgarian and Rusyn. In Interslavic, both options are equally valid.
Also declined like dva are the words oba and obydva „both”, „the two of”.
The numbers pet and up are inflected like nouns of the pattern kost. The subject always remains in the genitive plural. Examples:
The remaining numbers are declined as nouns, too: nula „zero” as a feminine noun (like žena), sto „hundred” as a neuter noun (like slovo), tyseč „thousand” as a masculine or a feminine noun (gen. tyseča or tyseči), milion „million” and miliard „milliard” (or „billion”) as masculine nouns.
Apart from the quantitative numbers listed above, there is also a category of so-called indefinite cardinal numbers, consisting of words like mnogo „much, many”, malo „little, few”, velje „a lot”, veče „more”, menje „less”, koliko „how many”, několiko „a few, some”, toliko „this/that many”, para „a few”.
Ordinal numbers, roughly speaking, refer to the place of an item in a row. In terms of grammar, these words are adjectives, and just like ordinary adjectives, they match with the noun they modify in gender, case and number. All ordinal numerals are inflected like adjectives with a hard stem (like dobry), except for trětji, which follows the soft declension (like svěži).
The ordinal numbers 1-4 are: prvy „first”, drugy or vtory „second”, trětji „third” and četvrty „fourth”.
The remaining ordinals, up to 99, are formed by adding the ending -y to the cardinal numbers:
pety „5th”, šesty „6th”, sedmy „7th”, osmy „8th”, devety „9th”, desety „10th”; jedinnadsety „11th”, dvadesety „20th”, etc.
The word for „100th” is sotny or stoty, the word for „1000th” is tysečny.
When a higher number forms a string, only the last member is modified to create an ordinal number: v tyseč devetsot osmdeset četvrtom godu „in the year 1984”.
Most Slavic languages use the ordinal numbers for fractions: dvě trětje „two thirds, 2/3”. However, for reasons of clarity if would deserve recommendation to use the following, universally understandable forms instead:
The word for „half” is pol (polovina, polovica). It can also be used as a prefix: pol-mrtvy „half-dead”.
All other forms are built by substituting the ending -y of ordinal numbers to -ina: tretjina „1/3”, četvrtina „1/4”, petina „1/5”, šestina „1/6”, sedmina „1/7”, osmina „1/8”, devetina „1/9”, desetina „1/10”. This works also for higher numbers: šestnadsetina „1/16”, sotina or stotina „1/100”.
They are inflected as feminine nouns. Thus, „7/38” is represented as sedm trideseti-osmin (the hyphen is meant to enhance clarity).
The cardinal and ordinal numbers are by far the most significant. However, the Slavic languages have other categories as well. Most of them are rarely used nowadays, or used only under particular circumstances, but they should be mentioned anyway for reasons of completeness.
The collective numbers refer to members a closed group, and can be translated as „a group of”, „-some”. The forms are:
dvoje „twosome, pair”, troje „threesome, a group of three”, četvero, petero, šestero, sedmero, osmero, devetero, desetero. This group also includes oboje „both”.
It can be used instead of a cardinal number in the following cases:
As can be seen from these examples, the noun following a collective numeral is always in the genitive plural, and if this noun is the subject of a sentence, the corresponding verb is in the 3rd person singular (neuter).
Collective numbers can also be used without a noun, in cases like: my oboje „both of us (m. + f.)”, zabava v troje „a party for three”.
The stems of the collective numerals can also be used as the base for multiplicative (or aggregative) adjectives: dvojny „double, twofold”, trojny „treble, triple, threefold”, četverny „quadruple, fourfold”. Less frequent, but not impossible, are higher numbers: peterny „quintuple”, šesterny „sextuple”, etc. To the same family also belongs jediny „only, single”.
With the prefix po-, these adjectives can also be turned into verbs: podvojiti „to double”, potrojiti „to triple”, početveriti „to multiply by four”. The opposite can be achieved with the prefix raz-: razdvojiti „to split in two”, raztrojiti „to separate into three parts”.
Another type of multiplicative numerals are adverbs, formed by adding the suffix -kratno to the cardinal number: jednokratno „once, one time”, dvakratno „twice, two times”, trikratno „thrice, three times”, etc. This suffix can be used for indefinite numbers as well: mnogokratno „many times”, několikokratno „a few times”.
Yet another group of numeral-based adjectives are the differential (qualitative, generic) numbers. They refer to a qualitative differentation of the items in question, and can be translated as „different types of”. An old form (based again on the collective numbers) is dvoj, troj (declined like the possessive pronoun moj).
However, more common and less confusing has the ending -aky: dvojaky, trojaky, četveraky, peteraky, etc.: dvojake knigy „two kinds of books”. This works also for indefinite numbers: kolikoraky „how many sorts of”.
When adverbialised, the meaning becomes something like „in X ways”: trojako „in three ways”.
Sometimes numbers are used as substantives, for example when we refer to a drawing or sculpture of a number, a bus, a radio or TV channel, a mark given in school, a banknote or a playing card. It can simply be translated as „a three”, „the five”, etc. In the case of the numbers 2-4, this is done by adding the ending -ka to the collective numeral stem, in the remaining cases to the cardinal number: jedinka, dvojka, trojka, četverka, petka, šestka, etc.
Alternatively, the ending -ica can be used, too, but this does not work in all cases: jedinica, petica, šestica, sedmica etc.
A somewhat special case is presented by the number of children from a birth: twins, triplets, quadruplets, quintuplets, etc.. The Slavic languages have different ways of expressing this: