The U.S. Constitution On-Line has been online in one form or another since 1995. In that time, I have have had a lot of questions asked of me about the Constitution. This page, and the ones it links to, are a compilation of the best of those questions.
This page lists the questions in subject order. You can also see the questions in their original, as-asked order and grouped by section of the Constitution. Also available are answers to questions about the structure of or details about the Constitution, and pointers to find the answer yourself.
Finally, after you explore the FAQ, you are welcome to ask your own questions, either by emailing the Webmaster or by posting a message to a Constitution Message Board. Either way, I will answer your question to the best of my ability, and your question may find its way here.
The National Archives has a Q&A page that is full of answers to questions about the Constitution's history and its mechanics. Check it out, too.