@JayDee
Well, that would have been interesting, to say the least. XD
@Shannor
You're right, "hai" can sound too terse on its own. Depending on the question, some other words should come after it, i.e.:
Hai, wakarimashita. (Yes, I understand.)
Hai, sou desu. (Yes, that's right.)
Hai, [positive form of the verb i.e. "yarimasu"] (Yes, I will.)
"Hai" is a rather polite way of agreeing. This would be okay if you're talking to someone older than you, or in a higher position than you, but if you're talking to your peers, friends, or someone younger than you, it would be more natural to agree by saying "un", "aa", "sou", or just the positive (short) form of the verb you're agreeing too. Though it could also depend on your personality and upbringing too.
Try and read the part entitled "14. Yes/No in Japanese" on THIS PAGE. (Find it via Ctrl+F). It gives a concise explanation of Yes and No in Japanese.