Why is "ha" (は) pronounced "wa" as a particle but not in words?
This admittedly tripped me up a lot while first memorizing Hiragana, I had to turn off the audio sound effects because the voice speaking out "wa" but needing to click or read it as "ha" was confusing me. Is there a reason for this and is that something I will actually need to keep in mind in practical application of Japanese speaking?
I typed all the stuff below without realising how much I was rambling. I decided to delete it because I'm lazy. It felt far too めんどうくさい just thinking about the effort it would take to turn into a sensible post. But, at the last second, I changed my mind... Instead I'll just post it "as-is" and leave it. People can just downvote it out of existence if this long mess of a post is a nuisance. No need for me to delete it myself. ^^
Warning: Virtually all the stuff I've written below I wrote completely off-the-top-of-my-head. I'm only a learner of Japanese like everyone else, except that I've been doing it too long and frequently waste hours looking into stuff like this rather than caring about getting fluent. Since I didn't reference anything while writing it, there might be things I've written that are wrong...
かは used to be the kana spelling for the word 川 (meaning 'river'). Historically it used to be pronounced more like "kapa". The kana spelling of this word changed from かは to かわ to make the spelling match the current pronunciation "kawa".
Many centuries ago, the kana は was pronounced as "pa". Its pronunciation has shifted over time. Now this kana is pronounced "ha".
However, even after the kana は changed to "ha", this was when pronounced on its own or at the beginning of a word that its pronunciation sounded like this. When the kana は came mid-word or at the end of a word, its pronunciation usually sounded like "wa". If you try to pronounce "kaha" quickly, it probably instead just sounds like it has a long vowel (like かあ "kaa").
So, the word 川 (かは) has become pronounced "kawa" rather than "kaha". And that's why when kana spellings were changed this was changed to かわ. These kinds of kana spelling changes happened for many many words.
Now, what about the は particle? Why is it pronounced "wa"?
Particles come at the end of a word (maybe "after" rather than "at", but they are attached to the end). So the は particle became pronounced as "wa" instead of "ha" for the same reason 川 became pronounced as "kawa" instead of "kaha".
Why didn't the は "wa" particle have its spelling changed to わ at the same time then? Why was there an exception made to leave its spelling untouched?
I think it was thought that changing the spelling of such a fundamental particle would cause a lot more problems, and therefore it wouldn't make sense to try to change it. Same with why they didn't change spelling of the へ "e" particle to え; or the を "o" particle to お.
It's not much of an issue changing 川 from かは to かわ, because, unlike the particles, most of the time it is written in kanji anyway.
If you read Japanese texts written before the 20th century, then you'll see plenty of words containing the kana "は" mid or end of word where it is "wa", not "ha".
庭 にわ used to be には "niwa"
沢 さわ used to be さは "sawa"
岩 いわ used to be いは "iwa"
表す あらわす used to be あらはす "arawasu"
回す まわす used to be まはす "mawasu"
It's true of other particles, too!
へ (towards) is pronounced more like "e"
を is pronounced more like "o"
は is probably the most drastic, but I think it's probably kept mainly as a way of being able to differentiate between words and particles. It doesn't make much sense to me, but then again, my Japanese teacher in my first year of university was a native who didn't understand it either xD
Interestingly, it is used in a few words, こんにちは and こんばんは namely. However, こんにち is an old reading for 今日 so it's more of a phrase! Same goes for the latter.
offtopic, but i love your icon @sheprd (i believe i know this artist from tumblr? long live mass effect!)