Why is the "hot" wire hot when the switch is off?
I also highly recommend Black & Decker's "Complete Guide to Home Wiring." It's so well-explained, it makes me feel like an electrician
Hope this helps!
The other part of this is whether the person who wired the switch and light knew what they were doing.
Never take a chance. Always kill the power at the breaker before doing any electrical work.
The book is an excellent reference.
If we asked enough questions about your situation, we could probably determine exactly what happened, but with the information so far, it is impossible to tell for certain.
One possiblility is that the electrician that wired the house used a switch leg to control the fan. In this case, one of the black wires in the fan would ALWAYS be hot unless the breaker is turned off. There would be no neutral in the switch box, only one always hot wire and one switched hot wire.
Regardless of what happened there, please remember that there is no substitute for safe wiring practices. Electricity is dangerous and many many people have been electrocuted when the "switch was off". If you are going to be performing your own electrical work, you should purchase a meter and/or other test devices to check the circuits you are working on. Some of these are very inexpensive and they will pay for themselves very quickly.
For simple wiring and testing, this board and others like it are a great place to ask questions, but remember your limitations and if you aren't sure about something, call an electrician.