Why is konnichiwa spelled こんにちは and not こんにちわ? Well, it is a bit of a long story, I fill in all the details in this slideshow. This presentation appears in the Memrise course 'Learn Basic Japanese' that you can find at the link below: http://www.memrise.com/course/80119/learn-basic-japanese/

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In the next level, you will learn the word for
hello, konnichiwa.
こんにちは
konnichiwa

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But you might notice that konnichwa is
'misspelled'
こんにちは
konnichiwa

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That's because this expression is actually a
shortening of a longer phrase.
今日は ご機嫌 いかがですか？
Konnichiwa gokigen ikagadesuka?
lit. (as for the topic of) today, how is your mood?

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In this longer phrase, the は particle is being
used.
今日は ご機嫌 いかがですか？
Konnichiwa gokigen ikagadesuka?
lit. (as for the topic of) today, how is your mood?

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This は particle is called the 'topic marking
particle', and as you might've guessed, it
marks the 'topic' of the sentence.
今日は ご機嫌 いかがですか？
Konnichiwa gokigen ikagadesuka?
lit. (as for the topic of) today, how is your mood?

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When the は particle is being used in this
way, it is pronounced 'wa'.
今日は ご機嫌 いかがですか？
Konnichiwa gokigen ikagadesuka?
lit. (as for the topic of) today, how is your mood?

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You can think of it this way, when it is with other kana it is
laughing 'ha' and having fun ...
はな
Ha! Ha! You are
so funny na!

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But by itself (as a particle), it is lonely.
あなた は つまらない。
anata wa tumaranai.
You are boring.
WAt happened to everybody?
I just want to be loved.

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Something similar happens with へ (he) and を (wo). They lose their
first sounds /h/ and /w/, so they are pronounced へ (e) and を (o).
へ
e
Eee! Sometimes I like
to be alone, okay?
を
o
O! No! I'm all
by lonesome!

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So in order to type the 'wa particle' you need to
type ha into the IME.
konnichiha → こんにちは

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Basically, long story short, sounds for
characters have shifted over time. The は
particle retained its original pronunciation.
は
Don't mess with a classic!

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If you are interested in learning about the
wonders of the は particle, I made a handy
video for you. Click below to check it out.