Teach Us, Teacher
When the letter for HA is pronounced as WA (Lesson 6)In the past, the topic marker, WA, was pronounced as HA, and was spelled as HA. Its pronunciation has gradually changed to WA, but the spelling remains as HA. The same can be said about WA in KONNICHIWA (hello). It is pronounced as WA, but is spelled as HA. Originally, this word consisted of the word which means "today" and the ancient topic marker, HA. And it was functioning as a common conversation opener, meaning "Talking about today." Now, KONNICHIWA has become a common greeting as one word.
In the case of DENWA O SHIMASU (I'll ring you up), the particle O was pronounced in a different way in the past. Now, it is pronounced as O. But when we write it, we still use the letter for the previous pronunciation.