こんにちは is probably the first word everyone learned in Japanese, it means “Good afternoon!” or just plain old, “Hello!” But have you ever considered what こんにちは literally means and why we use は for “wa” of “konnichiwa”?
The kanji for こんにちは is 今日は. 今日 is usually read as きょう, and both こんにち and きょう mean “today”. Compared to きょう, こんにち sounds more traditional, therefore, こんにち is often used in formal speeches, such as a speech in a wedding, a formal parties, or at a ceremony.
は of こんにちは is the topic particle は (wa). This is why we say “konnichi wa”, and not “konnichi ha”. こんにちは is an abbreviation derived from 今日はご機嫌いかがですか (=How are you today?). We see the exact same thing with the phrase 今晩は, or “Good evening.”
Sometimes, especially younger generations, people write this word as こんにちわ (or こんばんわ for ‘Good Evening’). The わ at the end here is an intentional mistake because it just makes the phrase a bit cuter and funnier!
Hello! Long time no see!
Hi! How’s it going?