Why would my instructor think using forums are very important?
The word "forums" is plural, so you might expect to use "are" for the verb, but "using forums" is singular, so use "is" for the verb: Why would my instructor think using forums IS very important?
Using forums is a good way to keep up-to-date on what you are trying to learn. By the time a book is printed, it may be extremely out-of-date. Learn the basics from the book and the instructor, then check forums to see what has changed.