Ten digits and four rank words build every number in Japanese. The
catch is that some of them change sound when they meet: 300 is
さんびゃく, not さんひゃく. There are five such changes and they are
marked wherever they appear below.
The digits
Four, seven and nine have two readings each. The
first is the one to use when counting; both are understood.
The ranks
Japanese groups its digits in fours, not threes —
which is why 10,000 gets a word of its own where English needs two,
and why 100,000,000 does too.
Where the sound changes
All five of them, in one place. Everything else
follows the digits exactly.
Counting up
Multiples of each rank. The marked ones are the
sound changes; the rest are the digit and the rank, joined.
Tens
Hundreds
Thousands
Counting things
Japanese does not count things bare. It counts
them with a word saying what kind of thing they are — and that word
changes sound after the same digits the ranks do. Pick the counter
by the shape of the thing: long, flat, small, bound, alive.
つつthings with no counter of their own — the general one
Its own set of words entirely, and it stops at ten.
人にんpeople
One and two are irregular; four is よにん, never よんにん.
本ほんlong thin things — pens, bottles, umbrellas, trains
枚まいflat things — paper, plates, shirts, tickets
Nothing changes. Rare, and worth enjoying.
匹ひきsmall animals — cats, dogs, fish, insects
個こsmall solid things — apples, stones, boxes
冊さつbound things — books, magazines, notebooks
台だいmachines and vehicles — cars, computers, washing machines
杯はいcupfuls and glassfuls — of tea, coffee, rice
歳さいyears of age
Twenty is はたち, which is not built from anything.
Marked readings are the ones that change sound. Everything unmarked is
the digit and the word for the rank, said one after the other.