The pronunciation of は・ひ・ふ・へ・ほchanged several times.
1. at the top of a word
（8th century ～ 16th century）
（17th century ～ now）
は［ha］・ひ［hi］→［çi］・ ふ［ɸɯ］・へ［he］・ほ［ho］※［ɯ］is non-rounded［u］.
2. except the top of a word
（8th century ～ 11th century）
（12th century ～ 16th century）
（16th ～ now）
※は and へ are words. But they are pronounced with the former word together. So they are treated as an exception of the top of a word.
※は・ひ・ふ・へ・ほ in traditional orthography were changed to わ・い・う・え・お according to the pronunciation after World War Ⅱ(1946). Actually わたしわ、がっこうえ are allowed for a while.
But は and へ are written in words come from particles now. The reason is for easy reading. When we read わたしわきのうがっこうえいきませんでした, it's too hard to catch the meaning at once. We use は and へ（and を）as particle markers to separate the sentence to words. So we write わたしは、こんにちは、これはこれは、がっこうへ（あなたを）even now.
July 5, 2015
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