Answer: Now we're getting somewhere - almost a real advice-column type of letter. When I was in Junior High School, around the time of the Lincoln administration, I wrote an advice column called "Ask Sadie Glotz" and I've never gotten over it.
But I digress.
First let me say that nothing is boring. When you are young you might imagine that some things can be boring, but that is simply the consequence of not knowing very much yet. When you look at things properly you'll find that the entire world is interesting when examined as it should be.
As an undergraduate I was a composition major, which meant studying theory. But "theory" in music is not quite like theory in particle physics. Most of what is meant by "music theory" is nothing more than learning how notation works, how chords and intervals are named, and so on. Later on you would study, if you're lucky, fascinating topics like the traditional art of counterpoint (which involves lots of technical 'recipes' for creating good effects). There actually is such a thing as a person who studies the theoretical aspects of musical experience and perception, but that's not what music theory courses are about, at least in the first couple of years. "Music Theory" courses are not rocket science, in short. What they teach you is really the language of music and how to communicate with others who know the lingo, so that you'll have a clue when someone refers to an augmented-sixth chord or to asymmetrical meter, or tells you that what is needed here is a secondary dominant.
As to composers randomly changing clefs in the middle of a piece, yes, this is their twisted idea of humor. No, scratch that. Composers, or transcribers, will change clefs from time to time because they feel it's easier to read in a different clef than to read lots of ledger lines. If the music moves out of the convenient range for one clef, the writer will frequently change to one that doesn't require as many ledgers. But most musicians don't need to know all the clefs. A viola player knows the alto clef and will sometimes see the treble clef, but there is no need for a viola player to know the bass clef. Cellos play in a wide range, so a cello player needs to know the bass clef as a beginner and later the tenor clef and on some occasions the treble clef. And so on.
Why are the parts for some instruments written in different keys? This goes to the acoustics of wind instruments: an instrument whose bore length is such that it naturally sounds Bb has a series of harmonics in the Bb series, beginning with the notes of the Bb major triad, so Bb major is the natural key of that instrument. The instrument is equipped with various holes or valves (or in the case of your trombone, a slide) to produce tones that are not part of its natural overtone series. Wind instruments traditionally were made in various sizes to play in different keys; the parts were transposed so that the sound would come out right.
These days wind instruments are more highly adapted to playing full chromatic scales, but the tradition of transposing their parts remains. And it's a good thing, too, for the instrumentalist. One instrument will be easier than the other one to play in a given key, so long as the part is transposed appropriately. Also, instruments with different fundamental pitch have different characters; so a composer might prefer one or the other. Transposing instruments are likely to remain with us, complicating your life and mine.
