I know that some people find music theory and standard notation to be nigh-impenetrable, so I'd like to share a resource that, based on a quick search, doesn't seem to have ever been mentioned in the SGS.
Keep in mind I'm no researcher—just another dude on the internet, so everything I say is purely based on my limited observation and is in no way meant to belittle anyone, because music theory is
hard.
Teoría is a site that includes both an introduction to music theory as well as exercises to practice ear training, sight singing, and the theory introduced by the tutorials. For understanding a good majority of everyday music, teoría has you covered in the first five modules; the sixth module on musical forms is mostly only useful for those studying classical repertoire—I say
mostly because the last section
Phrases, Periods, and Motives can be useful if you want to understand why certain song and verse structures work as they do.
I would say my only gripe about the material is that, at the time of writing—though I have just submitted feedback about this via the site's contact form—there seems to be no explicit mention of the fact that, in a traditional seven-note Western scale, whatever it may be, each base letter A–G must appear exactly once, something that explains partially why accidentals matter. It's an implicit convention that's really important for the sanity and consistency of standard notation but that,
because it's implicit, is probably overlooked or dismissed—a real pain if you've ever looked at the standard notation for a tab on UG.
I hope this helps someone. And take it slooow. Learning music theory is hard. I picked it up as a young kid through my piano lessons—group lessons by a wonderful teacher whose curriculum placed great emphasis on music theory—and it took several years to get to a reasonably high level. Granted, the pace was intentionally not too fast, likely so as not to overwhelm the students, but as we know, learning becomes harder with age. I would guess six months to a year or even a year and a half is probably the bare minimum for an adult working through this stuff.