Have you ever taken a physics class? I never studied physics in high school or college. My kids love science and math, so I was terrified to teach physics. I had no clue what I was doing and I could only rely on my husband so much - he had a job and didn’t have the time to help with physics every day.
My sons ended up teaching themselves physics in our home-school. They would sit down with the textbook in one hand and the answer key in the other hand. They read through everything and when they got stuck, they would look at the answer key. That was how they learned the concepts.
When it came time for a test, I would hand them the test and then take the answer key with me when I went grocery shopping. When I came home from grocery shopping, the test was magically done. Then it was my turn - I took my children’s answers and put them side by side with the answer key. To be honest, I didn't even know what the symbols meant, it was all like Greek to me. I just looked at each of their answers to make sure they looked exactly the same as the answers in the answer key.
Sometimes the answers would be different. When that happened, my children would try to convince me their answers were right and the textbook answer key was wrong. I always gave them the option of contacting the publisher or the author of the textbook to ask if the answer key was incorrect for that particular question. If they heard back that the answer key was indeed wrong, I would be more than happy to change their answer on the test and mark it as correct.
Most of the time, my kids didn't bother to contact the publisher and that was fine. They came to understand that if their answer didn't look exactly like the answer key, I would mark it incorrect, as that was my only method of marking. We also used the same strategy for other subjects I wasn’t familiar with, such as Math, Latin, and French. You, too can let your child self-teach and learn subjects far beyond your own understanding!