This video, via insidesynthesis, presents a method of understanding FM synthesis.
The discussion is tailored for those who know subtractive synthesis.
A lot of people struggle with understanding FM synthesis. Much of this confusion may come from the poorly designed interfaces of most FM synths.
Check out the video and let us know if it helps you make sense of FM synthesis!
See, too, this set of videos that looks at some of the Secrets of FM Synthesis.