Conclusion:
At this point you should have a solid understanding of the chords you can play in any given key, and the theory behind "how and why" these chords came about. Understanding this is a vital key to improvising and writing your own songs. Of course, you don't "have" to have any theory knowledge to write your own songs, but knowing this stuff will take a lot of the guess work out of writing.
But as always, use your ears and they will tell you what sounds good and what doesn't. All this theory stuff is just guidelines, you don't have to "stay in key at all times".... You don't even have to stay in key at all. However, it does help to know the rules before you break them.
* Next Lesson: Harmonic Conjugations