Why Gakkou Not Gakukou? - Sokuonbin 促音便

Basically, it's because of how the words are pronounced.

When you have a word like学校, it's easier to pronounce it asthan it is to pronounce it as. The tongue slips. People have been saying it that way for so long it's become official.This is similar to how rendaku 連濁 makes a死神 become死神.Besides, it also sounds better. I mean,突破 sounds like someone's name, it doesn't ring as nice as突破！！！

The sokuonbin happens in essentially three different cases.

When you have two morphemes (like two, but not always), and they're joined in one word, and the last syllable of the first morpheme and the first syllable of the second morpheme have hard, dry sounds, they end up becoming ainstead.

chi, tsu or ku, and the second morpheme starts with h, k or t, then it probably becomes a sokuonbin.

Also, chi and tsu also create sokuon sometimes when they come before s or h. And when h is preceded by the sokuon, a is added and it becomes p instead. (example: ho ほ becomes po ぽ)

Forced Word Boundary

When you have a morpheme which starts with one of the hard syllables above, like k, h, t or s, for example, and you use it as a suffix in a word, it's often the case you add the sokuon in order to let it show it's a separate morpheme in the pronunciation.

Suffixes

For example, the Japanese anime community has the custom of adding the suffix ko 子, "child," written with the kanji for musume 娘, "girl," to random words. When this happens, the suffix usually becomes kko っ娘.

bokukko 僕っ娘, because a bokuko 僕娘 sounds like it's a single word that mixes both things meanings somehow, when it's the one words with one suffix.

So a girl which uses the pronoun boku 僕 gets called a僕っ娘, because a僕娘 sounds like it's a single word that mixes both things meanings somehow, when it's the one words with one suffix.

You can also see this in the suffix ppoi っぽい. If you said just poi ぽい it'd sound like it's a single adjective. For example: nekopoi ねこぽい looks like one single word, it has less syllables than atatakai 暖かい, "warm," which is a simple adjective.

So, instead, an emphasis is put on the pronunciation of the boundary, nekoppoi 猫っぽい, "cat-like," so the listener or reader can perceive it's separate things.

Prefixes

This can also happen with prefixes, which is why ma 真 and shiro 白 become masshiro 真っ白.

Historic Changes

A last case is when a word's pronunciation simply changed through the years, and by years I mean decades or centuries, or who knows how long.

Such changes are difficult for someone like me to figure out, but I can at least tell you they exist.

For example, shippo 尻尾, "tail," is read as shippo in modern Japanese, but its parts are shiri 尻 and o 尾 respectively. You can tell that o 尾 was treated as if it was a ho and turned into a po. In the past, the word was read shirio 尻尾.

Te-forms

Most te forms of verbs, like totte 取って, actually come from literally connecting the verb to the te. This happens by using the "conjunctive form," the renk'youkei 連用形.

For example, connect toru 取る, "to take," to another verb would look like this: torikaesu 取り返す, "to take back." So when it's connected to the te, it should look like torite 取りて. However, after centuries of languagery, it's become totte 取って instead because of the pronunciation.

(in a similar but different process, yomite 読みて became yonde 読んで)

Join of Vowel

The use of sokuon also happens sometimes when the second morpheme is a vowel and the first is a hard syllable. When this happens, the vowel ends up being treated like a h syllable and turns into p.

For example, shiri 尻 and o 尾 become shippo 尻尾, "tail."

Examples

For example, if the first morpheme ends inor, and the second morpheme starts withor, then it probably becomes aAlso,andalso createsometimes when they come beforeor. And whenis preceded by the, ais added and it becomesinstead. (example:ほ becomesぽ)