Numbers and counting

Nine roots plus dek, cent and mil give you every number in Esperanto, and one spelling rule assembles them. Tens and hundreds are written solid: dudek is two tens, ducent is two hundreds. Everything else stays apart: dek du is ten plus two, and du mil (never dumil) is two thousand. So dudek du is 22 and cent dudek tri is 123. There is no thirteen-versus-three irregularity waiting for you, because nothing here is irregular.

In mi vidas tri homojn the noun picks up both endings and tri stays untouched. Speakers of Slavic and Romance languages expect the number to react, and it does not (only unu, used as a pronoun, can: la unuj kaj la aliaj, some and the others). The only numbers that decline are the ones that have stopped being numbers: ordinals are adjectives, la unuajn tagojn, and forms like duono and dekduo are nouns. Endings and suffixes build those out of a cardinal: -a, -o, -on-, -obl- and -op-.