A student may repeat a volume as often as desired for reinforcement. I especially would encourage students, parents, and teachers to contact me. If there are problems with the materials, instructional sequences, or anything whatsoever, please e-mail your comments. The nice part about the internet is that the materials can be updated and made available immediately - something that's just not possible with printed materials. Don't be discouraged when errors are made.
Errors are an important part of learning and an important part of life.
Remember that each exercise is an instructional sequence and not a test. Repeat the exercise or volume as many times as you like. The modern biolinguists seem to indicate that spoken language is strongly instinctual in nature. A person does not just mimic what has been heard but rather creates and composes a communication. Writing, however, seems to be more cultural than instinctive.