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You may encounter some Japanese pronunciation changes cases, for example:“koi” is “love”, “hito” is "person", and "lover" is “koibito”. The word "hito" is changed to "bito" for easier pronounciation. You can see the examples down below:
恋 koi + 人 hito = こいびと koibito (hi changed to bi) (lover)
手 te + 紙 kami = てがみ tegami (ka changed to ga) (letter)
国 koku + 家 ka = こっか kokka (ku becomes repeated consonant - small tsu - っ) (nation)
発 hatsu + 展 ten = はってん hatten (tsu becomes repeated consonant - small tsu - っ) (develop)
脱 datsu + 出 shutsu = だっふつ dasshutsu (tsu becomes repeated consonant - small tsu - っ) (escape)
つけっ放し tsukeppanashi (turned on electricity (fans, lights) but does not use): hanashi changed to panashi
日々: hibi (from days to days), the word 々 is only repeat the word before it, “hi” changed to “bi”
人々: hitobito (many people), the second “hito” changed to “bito”
国々: kuniguni (countries), “kuni” changed to “guni”
近頃: chika (near) and koro (such a time) become chikagoro = “recently” (koro changed to goro)
賃金: chin (wage) and kin (money) become chingin (payment), “kin” changed to “gin”
順風満帆: “advantageous like smooth sailing”, combines of “jun + fuu + man + han” and become jumpuumampan
それぞれ: sorezore (respectively)
青空=あおぞら: ao + sora = aozora (blue sky)
You can see that the pronounciations in some compound words or repeated words have some changes in the examples above. Why is that? The purpose is to make it easier for speaking and avoiding misreading. For example the word賃金, if we speak “chinkin” then it can be hard to pronounce while it is easier to pronounce it as “chingin”. Dakuon (濁音) - murky sounds are always easier to pronounce than Seion - clear sound, for example “sore zore” is easier to pronounce than “sore sore”.
Down here are some Japanese pronunciation change rules.
Some Japanese Pronunciation Change Rules
1) Compound words or repeated words: row "ha" (ha hi fu he ho) becomes row “ba” (ba bi bu be bo) which is the murky sound of row “ha”.


は ひ ふ へ ほ → ば び   べ ぼ

You can see that the murky sound is similar to clear sound, murky sound only has additional colon.

Eg: koi + hito = koibito, 日 hi + 日 hi = 日々 hibi (from days to days)
(2) Row “ka” changed to row “ga”
か き く け こ → が ぎ   げ ご
Eg: 近頃 chika + koro = chikagoro
(3) Row “sa” changed to row “za”
さ し す せ そ → ざ じ   ぜ ぞ
Eg: 矢印 ya (arrow) + shirushi (sign) = yajirushi, 中島 naka + shima = nakajima (person's name)
(4) Row “ka” if continued by another row “ka” then it changed to repeated sound
Eg: 国旗 (nation flag) koku + ki = kokki こっき instead of kokuki
(5) Row “ha” if followed by “tsu” つ then “tsu” becomes repeated sound while row “ha” becomes row “pa”
は ひ ふ へ ほ → ぱ ぴ   ぺ ぽ
Eg: 活発(かつ+はつ)=かっぱつ, katsu + hatsu = kappatsu
Row “ha” if followed by small “tsu” = “っ” then it changed to “pa”
Eg: つけっぱなし
(6) Row “ka” if followed by“n” (ん) then it changed to “ga”
Eg: 賃金=ちんぎん
(7) Row “ha” if go after “n” (ん) then normally it changed to “pa” (mostly) or row “ba” (less occurance)
Eg: 根本=こんぽん, kon + hon = kompon (core)
(8) Consonant “n” (ん) before “pa” or “ba” or “ma” then it will be pronounced as “m” instead of “n”
Eg: 根本=こんぽん kompon, 日本橋=にほんばし nihombashi, あんまり ammari