How to read Historical kana usage
●ゐ and ゑ, those are obsolete in Modern kana usage, are commonplace in Historical kana usage.ゐ is to be read as い, and ゑ is to be read as え.
ゐる→いる（居る）
こゑ→こえ（声） and so on.（Katakana of ゐ is ヰ, and that of ゑ is ヱ.）
●を can be used not only as a particle but also in other kinds of words.
を is to be read as お .
をばさん→おばさん and so on.
●ぢ and づ are used more often.
ぢ is to be read as じ, and づ is to be read as ず.
おぢいさん→おじいさん
まづ→まず（先ず） and so on.
●Some kana are read as different sounds instead of their original sounds under a certain condition. These phenomena are called Tenko(Change of call).
は, ひ, ふ, へ, ほ located in any place other than the top of a word are to be read as わ, い, う, え, お.
かは→かわ（川）
会ひます→会います
使ふ→使う
まへ→まえ（前）
おほい→おおい（多い） and so on.
Such couples of kana as あう, あふ, かう, かふ, さう, さふ, ・・・ are to be read in the same way as おう, こう, そう, ・・・.
あふぎ→おうぎ（扇）
行かう→行こう
さうです→そうです
ありがたう→ありがとう
たふとい→とうとい（尊い）
死なう→死のう
まうす→もうす（申す）
だらう→だろう
散らう→散ろう
言はう→言おう and so on.かうかう→高校
ざふきん→雑巾 and so on.
All the verbs such as 会ふ, 買ふ, 慕ふ, 嫌ふ are exceptions. They are read as あう, かう, したう, きらう in Modern Standard Japanese.
But in case of Ancient writings, they are generally read in the same way as おう, こう, しとう, きろう according to the general rule.
Such as きう, きふ, しう, しふ, ちう, ちふ, ・・・ are to be read as きゅう, しゅう, ちゅう, ・・・.
きうり→きゅうり
美しう→美しゅう and so on.えいきう→永久
じふじ→十時 and so on.
Such as けう, けふ, せう, せふ, てう, てふ, ・・・ are to be read in the same way as きょう, しょう, ちょう, ・・・.
けふ→きょう（今日）
でせう→でしょう and so on.けうしつ→教室
てふ→蝶 and so on.
●There are some other kinds of Tenko in Sound reading of kanji, in addittion.
Such as きやう, しやう, ちやう, ・・・ are to be read in the same way as きょう, しょう, ちょう, ・・・.
きやうだい→兄弟
たいしやう→大将
ちやうちやう→町長 and so on.
くわ, ぐわ are to be read as か, が.
くわし→菓子
マングワ→漫画
ゆくわい→愉快 and so on.
●Small letters are generally not used in Historical kana usage.
Large つ, や, ゆ, よ of Sokuon and Yoon are read in the same way as small っ, ゃ, ゅ, ょ.
あつた→あった
ちやんと→ちゃんと and so on.
●In case of Ancient writings, the following rule is to be added.
む of a suffix (particle auxiliary verb) is to be read as ん.
逢はむ→あわん
～せむ→～せん
ありけむ→ありけん
取りてむ→とりてん
給ひなむ→たまいなん
吹かむとす→ふかんとす
まかりなむずる→まかりなんずる and so on.
Historical kana usage:
When the hiragana was first created in the 9th century during the Heian period, they wrote it exactly as it was pronounced. After that, the pronunciation changed with the times, but they have been writing it for a long time just as they did at the beginning, as much as possible.
And so now we read it out with modern pronunciation.There are some exceptions and additions to the above rules.