The above reply nails it. You can't as a beginner just read one book and expect to grasp the theory completely. You have to undertake extensive reading. If you're really struggling then here's what to do.Just go onto Youtube and you'll find some really good channels that explain the theories in 8-10 min with examples. Then go back to your book and voila! Everything starts making sense. It actually works.

P.S. Important thing is that you stick to one theory at a time.