Understanding the big picture when it comes to music theory can be difficult. To help, contemplate the doily. A piece of music is a lot like that doily. Many types of stitches and knots are used to construct a whole doily, with perhaps hundreds of different techniques being responsible for the invention and evolution of each piece of the pattern.
However, unless you’re really into constructing doilies yourself, you really don’t care, or care to know, that more than 600 years of history and shared knowledge went into constructing that round, lacy thing your grandmother sets under her coffee cup to protect the woodwork.
The casual pianist who pounds out Christmas carols once a year probably doesn’t think much about how every single piece of written music contains more than 7,000 years worth of speculation, theory, and technique. The notation of pitches and rhythm on the musical staff, the concepts of harmony and melody, and even the tuning system used to synchronize instruments has been a long, massive undertaking of science, art, and aesthetics.
Understanding and notating music has been as much a part of the human experience as the development of the written word. In many ways, knowing how form works makes composing music extremely easy. After all, the pattern’s already there — you just have to fill in the blanks. The real challenge is making your particular sonata, fugue, or concerto stand out from every other piece of music written in that form.