質問: こんにちは? why is it こんにちは and not こんにちわ?
回答: こんにちは http://www.italki.com/entry/554151 the pronunciation of は・ひ・ふ・へ・ほchanged several times. 1. at the top of a word (~7th century) 波(→は)pa]・比(→ひ)[pi]・不(→ふ)[pu]・部(→へ)[pe]・保(→ほ)[po] (8th century ~ 16th century) は[ɸa]・ひ[ɸi]・ふ[ɸu]・へ[ɸe]・ほ[ɸo] (17th century ~ now) は[ha]・ひ[hi]→[çi]・ ふ[ɸɯ]・へ[he]・ほ[ho]※[ɯ]is non-rounded[u]. ※ひゃ[ça]・ひゅ[çu]・ひょ[ço]、ファ[ɸa]・フィ[ɸi]・フェ[ɸe]・フォ[ɸo] 2. except the top of a word (~7th century) 波(→は)[pa]・比(→ひ)[pi]・不(→ふ)[pu]・部(→へ)[pe]・保(→ほ)[po] (8th century ~ 11th century) は[ɸa]・ひ[ɸi]・ふ[ɸu]・へ[ɸe]・ほ[ɸo] (12th century ~ 16th century) は[wa]・ひ[wi]・ふ[wu]=[u]・へ[we]・ほ[wo] (16th ~ now) わ[wa ※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.