I guess we could start at how there are only five vowel sounds in Japanese, but I suppose you know that (or read it on Firaga44's talk page under "Kana...".). Anyways, this month's goal is greetings (XD KH:CoM Manga reference). *ahem* There are three well-used greetings used at different times of the day.
おはよう = good morning.
こんにちは = after morn (usually 11 AM), before evening.
こんばんは = good evening/night when you're meeting someone.
However, we tend to shorten down greetings, or practically everything in Japanese (probably because it's a mouthful and it's inconvenient). So...
おは = a shortened "おはよう".*
こんちは = a laid-back version of "こんにちは".*
There is no shortened version of "good evening."
Some other greetings are:
オッス = Hey*
よっ/よう= Yo.*
ひさしぶり = Long time, no see.
*=Don't be using these ones towards businesspeople, especially if you're going to be forcing them to hire you as a videogame tester!