langula
How numbers work in Japanese
Japanese has two number systems. The Sino-Japanese numbers いち, に, さん … じゅう are used for almost everything: prices, times, dates and maths. There is also a native Japanese set from one to ten (ひとつ, ふたつ, みっつ … とお) used for counting things in general.
Four, seven and nine each have two readings: よん or し, なな or しち, きゅう or く. よん, なな and きゅう are preferred when counting and in larger numbers, partly because し can also mean 'death'. For clock times, however, you say よじ (4 o'clock), しちじ (7 o'clock) and くじ (9 o'clock).
From 11 the system is fully regular: 十一 (じゅういち) is 'ten one', 二十 (にじゅう) 'two ten', 二十一 (にじゅういち) 'two ten one'. With hundreds and thousands the first sound sometimes changes: 三百 さんびゃく, 六百 ろっぴゃく, 八百 はっぴゃく, 三千 さんぜん, 八千 はっせん.
Large numbers are grouped in fours, not threes: 10,000 has its own word, 一万 (いちまん). 100,000 is 十万 (ten まん) and a million is 百万 (a hundred まん).
When counting objects you attach a counter suited to the kind of thing, such as 本 (ほん) for long objects or 枚 (まい) for flat ones. Ordinals are formed with 番目 (ばんめ): 一番目 means 'the first'.