It seems like there's a continuing series over here. :)
If you collects some of my answers since the beginning of the beta, you can get a partial answer to your question, but that's not a full answer to your question, and frankly, a full explanation would require explaining the entire pronunciation system of the historical kana usage (歴史的仮名遣い), as it is called, and this should hopefully go one day into the Wikipedia article, not into a Stack Exchange answer. :)
Still, I can give you a few points that should explain the general gist of that. But first, we have to see how Old Japanese (the Japanese spoken circa 800 CE) sounded:
The pronunciation of all columns was more persistent. For instance, ち was probably pronounced TI instead of CHI, and じ was pronounced ZI instead of JI.
entire H-column (は行) was pronounced (persistently) with the sound /p/. i.e. は was PA, へ was PE, ふ was PU, etc.
There were a few kana symbols that represent sounds that don't exist anymore: を represented WO, ゑ represented WE and ゐ represented WI. WO now represents the sound O, which but the separate letter still remains and it is used more or less exclusively for the particle を. The other two letters (ゐ and ゑ) have been removed from the alphabet in the reform of 1946.
During the time that passed since then, the spoken Japanese language has naturally changed quite a bit, and many sounds have been either lost or transformed:
The consonant /p/ became /f/ and later /h/ (though the pronunciation /f/ remained before the vowel /u/.
Between vowels, however, the consonant /p/ usually changed to /w/ and merged with the existing 4 sounds in the W-column (わ・ゐ・ゑ・を), while ふ merged with う (since there was no separate letter or sound for WU).
Later still, the sound /w/ disappeared anywhere except for before the vowel /a/, so ゐ・ゑ・を merged with い・え・お.
At more or less the same time, there were some vowel contractions (monophthongizations in linguistic techspeak) that caused adjacent pairs of vowels to be pronounced like one (usually long) vowel. These included the following:
- あう -> おお (remember that also includes わふ -> わう -> をお -> おお etc.)
- おう -> おお (including をふ, をう and おふ)
- えい -> ええ (including ゑい, えひ, etc.)
- いう -> ゅう (e.g. 十, which was
jip in Middle Chinese and じふ in Old Japanese, became じゅう).
- えう -> ゃう -> ょう (e.g. けふ in 今日 become きょう)
It should be noted that small kana wasn't used (at least most of the time) in the old kana usage, so じゃない would have been written じやない, and あった would have been written あつた.
While all these changes occurred, the spelling conventions remained mostly unchanged. There were some attempts at reform during the Meiji period, but besides throwing away the Hentaigana, they didn't work out. If we look back at the situation before WWII, the pronunciation was more or less the same as today, but words were being spelled as if they were pronounced in Old Japanese, which was quite confusing since a word pronounced in one way could be written in several different ways even when it was written in kana (not to mention kanji :)), and you had to know which of them was the correct one.
Some interesting examples are:
- オウ (応), ワウ (王), アフ (凹), ヲウ (翁), ワフ (奥). All are now normalized as オウ.
- All the verbs now ending in う used to end in ふ, 闘う・戦いたい was 戦ふ・戦ひたい.
(note that the spelling おう, which is pronounced the same as おお, along with the spelling えい, which is pronounced the same as ええ are the only two old spellings that remain in use).
You can find more examples and information here (although the article at this link has a few minor mistakes).
