A rather peculiar feature in Finnish is that a noun associated with a cardinal number is not in plural as in English and many other languages (e.g., “five apples”), but in partitive singular, e.g. viisi omenaa. This is not indicated in writing when the noun is expressed with a symbol or an abbreviation; e.g. we write 5 € but read viisi euroa. Similarly, 100 m is read sata metriä.
The partitive rule applies only when the expression as a whole is in the nominative, e.g. when appearing as a subject of a sentence. The expression viisi omenaa can be seen as consisting of a numeral (viisi), used like a noun, with a normal noun (omena) as its attribute in the partitive. In other case forms, however, the numeral is used like an adjective: there is congruence between it and the noun. Examples:
When the noun is expressed with a symbol, the suffix is written only after the symbol, but it is read for the numeral, too. We write 5 €:lla but read viidellä eurolla (for 5 euros, with 5 euros). In this context, “symbol” means a special character (like €), an identifier (like EUR, the currency code for the euro), or an abbreviation (like e, the Finnish abbreviation for the euro).
By convention, the suffix is also written when the expression as a whole is in the partitive. This makes it possible to distinguish between viisi euroa and viittä euroa, though in an unnatural way: 5 € and 5 €:a. In the latter expression, the suffix indicator :a really means that the preceding numeral is to be read in the partitive. Yet, the partitive written is that of euro|a, not that of the numeral, viit|tä.
Historically, an expression like viisi miestä can be understood as meaning “five of the collective of men”. However, it has become a grammatical construct. As an expression for “n of….” (where n is a number), we use the elative (stA case) plural, not the partitive. Note the difference between expressions like the following:
In the former type, both the numeral and the noun are inflected, e.g. viidelle miehelle (to five men). In the latter type, only the numeral is inflected, e.g. viidelle miehistä.
The numeral yksi (one) is the only exception to the rules above. We say yksi omena, using the nominative for both words. The expression is inflected so that both words take the same case form, e.g. yhdellä omenalla. Thus, yksi is used like an adjective in all case forms.
Numerals expressing fractional numbers have the noun in the partitive singular, too, e.g. puoli omenaa (half an apple). The same applies to decimal numbers, e.g. yksi pilkku viisi metriä, normally written as 1,5 m.
Even for the numeral nolla (zero) and the synonymous expression ei yhtään, the noun is in the partitive, e.g. nolla pistettä (zero points) or ei yhtään pistettä (no points).
The partitive is also used with words expressing inexact amounts such as pari (a couple of) and monta (many), e.g. pari omenaa, monta omenaa. The partitive, but in plural, is used with the words joukko (a set of), muutamia (some), paljon (many, plenty of), vähän (a few, a little), e.g. paljon omenia. Even these take the partitive singular for mass nouns, e.g. vähän vettä (a little water).
The word pari mentioned above means “two or three” when used to express a vague amount. It is also used as a normal noun meaning “pair”, and then the associated noun is in the partitive plural, e.g. pari kenkiä (pair of shoes), though often a compound like kenkäpari is used instead.
With plurale tantum words (nouns that are always used in plural), the number is in plural. It is in the nominative when the expression as a whole is in the nominative, e.g. kahdet häät (two weddings), but inflected as needed by the context kaksissa häissä (in two weddings).