Have you ever wondered why the topic particle は being pronounced as わ? Then this short text is for you!
In the early history of Japanese, the modern HA row consonants (HA, HI, FU, HE, HO) were pronounced with P as PA, PI, PU, PE, PO.
So mother was pronounced as PAPA .
However, in the Nara period (710 to 794) the P sound shifted to softer F (FA, FI, FU, FE, FO).
The same phenomenon happened in Indo-European languages (Grimm’s law) for example:
Proto-Indo-European *pṓds (foot) changed to *fōt- in proto germanic languages and eventually to foot (in English), Fuß in German and Fod in Danish.
For example, mother (母（modern HAHA）was pronounced now as FAFA).We can still see remnants of this in the pronunciation of ふ (FU).In the Heian period (794 to 1185) another shift happened, the F sound changed to W, but ONLY when it followed a vowel, so it wasn’t used at the beginning of the word.
Example:母(FAFA) started to be pronounced as FAWA.川 (KAFA) started to be pronounced as KAWA. (川 - river)This sound change is the reason why the particle は is pronounced as わ, more about it a bit later.
Eventually, in the Edo period (1603 and 1867, Edo is the original name of Tokyo) when people from various areas of Japan started coming to the Edo resulting in various dialects intermixing, and the F sounds started to be pronounced as H, resulting in modern pronunciation. Of course, ふ was the exception. So 母（FAWA）shifted to modern HAHA.However, the orthography did not change and even though words were pronounced in a new way, the old kanas that represented old pronunciation were used which resulted in a linguistic wild west, luckily, most often this was the case for words written with kanji.
(By the way, another interesting sound change in the Edo period was the change of Ri to I in some words, like ござります→ございます。)
And eventually, soon after World War 2, the Japanese reformed writing (one of the propositions were complete abolition of kanji, by the way), so that it would reflect the actual pronunciations so 川(かは) now was written as 川（かわ）. However, the particles were excepted because many felt that changing these exceedingly common spellings would confuse readers.The same reform retained the historical writing of particles へ and を, and also 当用漢字表（とうようかんじひょう) touyou kanji list (lit. “List of kanji for general use”) were made (the precursor of modern 2136 常用漢字 (jouyoukanji).
To sum up, は is pronounced as わ because the transcript reflects obsolete now pronunciation that was not changed during the language reform.
PSThe わ used at the end of the sentence (the one used for exclamation) comes from the topic particle は, yet it is written as わ in modern Japanese
