Today let's discuss how to count numbers in Japanese language.
Japanese is quite rational language, I hope, but I must say that Japanese has too many ways to say numbers.
Let me show you some examples.

Checker counting board
(1) pure numbers

English / Kanji(Hanzi, Chinese Ideographs) / Pronunciation in Hiragana / Pronunciation in Roma-ji (English like description)
One / 一 / いち / Ichi
Two / 二 / に / Ni
Three / 三 / さん / San
Four / 四 / よん / Yon
Five / 五 / ご / Go
Six / 六 / ろく / Roku
Seven / 七 / なな / Nana
Eight / 八 / はち / Hachi
Nine / 九 / きゅう / Kyuu
Ten / 十 / じゅう / Juu

Following numbers have the variants of pronunciation.

Four / 四 / し / Shi

Seven / 七 / しち / Shichi

Nine / 九 / く / Ku

i.e.
One litter / 一リットル / いちりっとる / Ichi rittoru
Two meters / 二メートル / にめーとる / Ni meetoru

It is interesting that if you count up, you rather say:
Ichi, ni, san, shi, go, roku, shichi, hachi, kyuu, juu

But if you count down, you rather say:
Juu, kyuu, hachi, nana, roku, go, yon, san, ni, ichi

Don't ask me why.
Somehow, it seems more natural to change the pronunciation.

(2) pieces of countable object

One piece / 一つ / ひとつ / Hitotsu
Two pieces / 二つ / ふたつ / Futatsu
Three pieces / 三つ / みっつ / Mittsu
Four pieces / 四つ / よっつ / Yottsu
Five pieces / 五つ / いつつ / Itsutsu
Six pieces / 六つ / むっつ / Muttsu
Seven pieces / 七つ / ななつ / Nanatsy
Eight pieces / 八つ / やっつ / Yattsu
Nine pieces / 九つ / ここのつ / Kokonotsu
Ten pieces / 十 / とお / Toh

i.e.
Three rice cakes / お餅が三つ / おもちがみっつ / Omochi ga mittsu
Four rice cakes / お餅が四つ / おもちがよっつ / Omochi ga yottsu

(3) glass of beer

One glass of beer / ビールが一杯 / びーるがいっぱい / Biiru ga ippai
Two glasses of beer / ビールが二杯 / びーるがにはい / Biiru ga nihai
Three glasses of beer / ビールが三杯 / びーるがさんばい / Biiru ga sanbai
Four glasses of beer / ビールが四杯 / びーるがよんはい / Biiru ga yonhai
Five glasses of beer / ビールが五杯 / びーるがごはい / Biiru ga gohai
Six glasses of beer / ビールが六杯 / びーるがろくはい / Biiru ga rokuhai
Seven glasses of beer / ビールが七杯 / びーるがななはい / Biiru ga nanahai
Eight glasses of beer / ビールが八杯 / びーるがはっぱい / Biiru ga happai
Nine glasses of beer / ビールが九杯 / びーるがきゅうはい / Biiru ga kyuuhai
Ten glasses of beer / ビールが十杯 / びーるがじゅっぱい / Biiru ga juppai

It is interesting that the same character 杯 has various pronunctiations : hai, pai, and bai.
This is the phenomenon called 音便 (おんびん, on-bin)that makes the pronunciation EASIER.

The more intersting thing is that ビールが一杯 has two meaning: one glass of bear and many glasses of bear.

(4) days (length)

one day long / 一日間 / いちにちかん / ichi nichi kan
two days long / 二日間 / ふつかかん / futsuka kan
three days long / 三日間 / みっかかん / mikka kan
four days long / 四日間 / よっかかん / yokka kan
five days long / 五日間 / いつかかん / yokka kan
six days long / 六日間 / むいかかん / muika kan
seven days long / 七日間 / なのかかん / nanoka kan
eight days long / 八日間 / ようかかん / yohka kan
nine days long / 九日間 / ここのか / kokonoka kan
ten days long / 十日間 / とうか / tohka kan

(5) day of a month

the first of May / 五月一日 / ごがつついたち / gogatsu tsuitachi
the second of May / 五月二日 / ごがつふつか / gogatsu futsuka

The third ~ the tenth are the same way as days (length).
The first (tsuitachi) is only one exception.

(6) month of a year

January / 一月 / いちがつ / Ichigatsu
February / 二月 / にがつ / Nigatsu
March / 三月 / さんがつ / Sangatsu
April / 四月 / しがつ / Shigatsu (Caution: Yongatsu sounds weird! Somehow!)
May / 五月 / ごがつ / Gogatsu
June / 六月 / ろくがつ / Rokugatsu
July / 七月 / なながつ / Shichigatsu (Caution: Nanagatsu sounds weird! Somehow!)
August / 八月 / はちがつ / Hachigatsu
September / 九月 / くがつ / Kugatsu (Caution: Kyuugatsu sounds weird! Somehow!)
Octover / 十月 / じゅうがつ / Juugatsu (Caution: Tohgatsu sounds weird! Somehow!)
November / 十一月 / じゅういちがつ / Juuichigatsu
December / 十二月 / じゅうにがつ / Juunigatsu

I think, for the months' name, Japanese is more rational than English because most English names are from Greek God/Goddess.
July and August are from Emperor's names.
September, Octover, November and December are from numbers, but they are all shifted.
They should be 7th, 8th, 9th and 10th month.

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Anyway, you might think Japanese numerics are so intersting or complicated.
Please try to remember them.