Ordinal Numbers

The ordinals answer “which one in order?”, and grammatically they are not numbers at all - they are ordinary -ος / -η / -ο ADJECTIVES that agree with their noun in gender, number and case: ο πρώτος όροφος, η πρώτη μέρα, το πρώτο παιδί. The first ten are πρώτος, δεύτερος, τρίτος, τέταρτος, πέμπτος, έκτος, έβδομος, όγδοος, ένατος, δέκατος - and only πρώτος, δεύτερος and τρίτος have to be learnt as separate words; the rest are visibly built on the cardinals you already know (πέντε → πέμπτος, δέκα → δέκατος). One rule covers the whole set and it is a comfortable one: the tonos NEVER moves. Because every ending is a single syllable the word never changes length, so δεύτερος → δεύτερου, έβδομος → έβδομου, δέκατος → δέκατου - unlike the -ος NOUNS (άνθρωπος → ανθρώπου), which do shift. Two spellings to watch: όγδοος takes the feminine όγδοη (not «όγδοα», which is its neuter plural), and the ninth is ένατος with one ν.