#1
Alright, so I've been playing guitar for a couple of years now and I'm trying to learn some music theory. I know some of the basics, but what are some subjects that I absolutely must know as a guitarist?
#3
Nothing. The only thing you need to know as a guitarist is how to put your fingers on the frets and strum. A guitarist is simply someone that plays guitar on a regular basis. Theory isn't compulsory.

Although, if you told us why you are learning theory we could tell you what branch of theory you would need.
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        L.