Why does "fu" changes to "pu" while it's not started with "h"?

Question

Supposedly, if the consonant starts with "h", it changes to "p", right? But "fu" doesn't have an "h". Then why does "fu" change to "pu"?

Answer 1

The //h// line in the kana are a bit odd. This is mostly due to history.

Derivation

Way back at the beginning of Japanese history -- and by that, I mean when we first start seeing text in the Japanese language, in the 700s or so -- the language had a //p// sound, and the five syllables we know today as [は]{ha} [ひ]{hi} [ふ]{fu} [へ]{he} [ほ]{ho} were instead pronounced as something like //pa//, //pi//, //pu//, //pe//, and //po//.

Over time, the initial //p-// sound lenited or softened, shifting from a hard //p// to something more like an //f// sound, producing //fa//, //fi//, //fu//, //fe//, and //fo//. Linguists think that this shift was probably complete by the Heian period (794–1185).

When this //f// sound occurred in the middle of a word, it lenited even further, shifting to more of a //w// sound. Particles are treated phonetically (sound-wise) as if they were suffixes, and this is why the particle は is pronounced //wa// today. In certain combinations, the //w// disappeared -- initially just before //u//, since //wu// is hard to pronounce. This is the same //w// that reappears in certain verb conjugations: modern verbs like [あう]{au} used to be [あふ]{apu} in the ancient language, and after the //p// → //f// → //w// shift, //awu// became //au//, but //awa// as in あわない still remains. Over time, the other //w// sounds -- //wi// and //we//, and to some extent //wo// -- all flattened out, losing the //w// and becoming just the vowel sounds. Only //wa// and //wo// are left, but you'll only hear //wo// when someone is deliberately emphasizing the pronunciation. Otherwise, を is basically pronounced as //o//.

Where this sound didn't shift to //w// and/or vanish, the //f// shifted further to //h// -- in all cases except before //u//. This is due to the biomechanics of Japanese pronunciation, where the //u// sound is made with the lips closer together than in English (or probably Spanish too for that matter). When pronouncing the Japanese //u//, it is actually physically difficult to pronounce //hu//: a bilabial //f// sound, technically [[ɸ]], happens naturally.

Alterations

Voicing to a //b//

Although the original //p// sound changed to //f// in the Heian period, a //p// sound re-emerged shortly thereafter. Voicing of a //p// clearly becomes //b//, and the 濁点【だくてん】 (dakuten, literally "muddy dot") or ゛ mark, also sometimes called a 点々【てんてん】 (tenten, literally "dot dot"), is used to mark voicing for any kana that can be voiced. So we have [た]{ta} plus the tenten becomes [だ]{da}, for instance. Even though ほ is now pronounced //ho//, the same voicing rule applies as if it were a //p//, so adding the tenten produces [ぼ]{bo}.

Hardening to a //p//

But, again, the はひふへほ sounds are historically derived from //p//. In order to clearly spell a //p// sound, Japanese writers invented the 半濁点【はんだくてん】 (handakuten, literally "half-muddy dot") or ゜ mark, to indicate where the はひふへほ kana should be read with the //p// sound.

Romanizing

Your question mentions the letters "h" and "f". Remember that Japanese was never written using the Latin alphabet -- so any spellings using Latin letters should only ever be considered as learning aids.

Since the Latin alphabet is new to Japanese, different writers using romaji have different ideas about which letters to use. Different romanization systems have developed to try to represent the Japanese language in Latin letters. Some try to use letters that are closer to how an English speaker might pronounce Japanese sounds (like using ⟨ fu ⟩ to spell [[ɸu]], since this sounds closer to "fu" for English speakers), and others try to use letters that are closer to some consistent theory of phonemic (sound-meaning) representation (like using ⟨ hu ⟩ to spell [[ɸu]], since most of the はひふへほ kana start with an //h// phoneme).

See also the Wikipedia article at Romanization of Japanese.

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