I will be going to Japan on business and would like to know if I say "HI" to a Japanese person will they think I'm saying "Hello" or "Yes"?

Although the Japanese word for "yes" is "hai", I assure you that if you do not speak Japanese there is no possibility they will confuse your "hi" for their "hai". Try learning a couple of the basic Japanese greetings, or at least just use "hello" and a smile - they'll understand perfectly well.

So if a Japanese person walked up to you on the street in your hometown and said "Konichiwa", would he assume you knew that he was saying "Hello, how are you?" or would he be thinking "I'm a visitor to America and I can't believe this idiot doesn't understand my what I'm saying! Isn't everyone suppose to know Japanese?" Please avoid being an Ugly American overseas and learn a few Japanese words to get by.

OK here we go:
good morning-OHAYO GOZAIMASU
good afternoon-KONNICHIWA
good evening-KONBON WA
good night-OYASUMI

And because my son teaches kindergarten in Japan, I could also tell you how to say fart and booger ;o)

BTW konnichiwa starts at about 10 or 11 in the morning, not after noon.
Also Hi sounds different to a Japanese person than Hai (yes)

ttt-are you Japanese? because my son lives in Japan and this is what he has learned, Konichiwa is "good afternoon" Ohayo (sounds like Ohio) is good morning.So wouldn't afternoon start around noon and after?