A stupid conversation today illustrates the many uses of the word 'hai'. This happened at the McDonalds (yes, I go there too much :S).
Cashier: will you be eating here?
Me: hai (yes)
Cashier: hai (I understand). What would you like to order?
Me: I'd like a large coca cola zero please.
Cashier: hai (I understand). that'll be 240 yen.
Me: hai (hands over money)
Cashier: hai (I accept your money)
Cashier: hai (and I return your change)
Me: hai (I accept the change and the receipt)
I hope this illustrates the silliness of Japanese. One word does everything. You can also the phrase 'hai?' meaning "I don't understand what you're saying". It all depends on the tone.
Speaking of silly Japanese, it's been bothering me for a while now, but Japanese are completely lab-rat-trained to stop in front of a railroad crossing. Always, no exception. Even if you can see perfectly that there is not a train in sight within 10 kilometers, a Japanese person will still stop in front of the railroad crossing, look both ways, and then slowly proceed to cross. Now imagine a hundred cars doing that and the kind of traffic jam this causes. I saw two of those traffic jams today.
Oh, and this story reminds me of another silly Japanese thing: a long while back when I took a trip by train to Hokkaido I booked a hotel in the middle of nowhere and was picked up my the owner by car, who drove me to the hotel. In the middle of nowhere there were traffic lights, signaling red. We had a perfect view of the road to cross and there was not a car in sight as far as the eye could see. The hotel owner waited in his car for at least 5 minutes, then said 'chotto matte kudasai', please wait, jumped out of his car, ran to the sidewalk and pushed the button that pedestrians need to push to cross, then ran back to his car and finally crossed when the light turned green. Japan is probably the only country where this would happen.