What is it
Having come from a warm water, bass bugging environment, when I first got into this game, I was totally clueless about what all of these different flies were, what they were supposed to imitate, and how and when to use them. They had names like Quill Gordon, Adams, Royal Coachman. What the hell kind of fly is that? I had heard of house flies, horse flies, dragon flies, but a Royal Coachman????
Then, while taking a fly tying class, I got a brief education in entomology. The study of bugs and their life history. At least as far as a trout is concerned. Along the way I discovered that a lot of these flies were given the names of their creators, rather than the bug they were designed to imitate.
The easiest part to pickup on was when to use a particular fly. Just be observant. If there are little black gnats flying around your face, if there are these little things with gray wings floating down the river and flying away, well,,,start there. Shake the leaves of the bushes along the stream bank and see if anything flies out of there. Watch for rises. Learn to recognize the difference between the way a trout rises to a caddis vs. a mayfly, vs. an emerger.
Then there are the times when there are no rises.
I fish because the voices in my head tell me to