Here is a basic explanation of ga and wa when marking subjects/topics of sentences:
ga can be used kind of like an 'of all things'. For example, I (of all people) eat --> わたしがたべます。
wa is more general: I eat --> わたしはたべます。
Ga can also be used to mark topics, such as in lesson 3.1 when Kenny used "ga" to indicate that his idea was the topic of the sentence (ideas, feelings, that sort of thing)
Wa on the other hand is used more for subjects such as in 3.1 when it marks the car as the subject of the sentence (tangible objects)
As you can see, these are pretty basic, and I'm sure Pascal-san has more in-depth explanations. However, I hope these help for now!