Sonntag doesn't just teach you to pass a test; he teaches you to diagnose energy systems.
Buy the International Version (usually cheaper) and find the Solution Manual ( only to check your work after you have genuinely tried). Thermodynamics is a contact sport. You cannot learn it by watching; you have to do the problems with pencil and paper.
Open to Chapter 3 (Properties of a Pure Substance). Find the diagram for $P-v-T$ surface. Stare at it for 5 minutes. If you understand that diagram, you have already beaten half the course.
Have a specific Sonntag problem driving you crazy? Drop the chapter and problem number in the comments below.
If you are an engineering student, you have probably heard the name Sonntag whispered in hallways with a mix of respect and fear. The textbook Fundamentals of Thermodynamics (often just called "Sonntag") is the gold standard for engineering thermodynamics. It is also famously dense.