Those of us who have completed the series of ABRSM examinations in piano will have had the experience, most likely, of cramming enough music theory into their heads to pass the grade 5 music theory examinations and then to move on as swiftly as possible. What is gained is a piece of paper that informs us that we understanding the basics of music theory, but little more. We may understand the circle of fifths, key and time signatures, simple and compound musical time, and rudimentary chord progressions, but, at that point, no one would expect us to subject a musical score to serious depth of analysis.
Music theory helps us to understand the pieces we learn
There is more to learning a piece of music than simply becoming acquainted with the notes on the page -- some musicians, indeed, do not read music. The notes can get us some way, and good instincts a little further, but to realize a piece in a way commensurate with the style and period of the composer requires deeper study. Music theory can help us to achieve this aim.
To anticipate what is coming next and to frame it correctly when we perform it, we need to grasp the deeper meanings of the structure of the piece, its key progressions, how the composer is utilizing suspense and resolution, modulations, and tuplets among other devices to produce a certain effect in his music. And, for the performer, this interaction between music theory and music practice can help determine how and where he adds expression or restrains that instinct, or whether he voices one part or another of some particular chord.
Once we grasp, through music theory, why a composer is doing what he is doing, we can use this information to improve the musicality of our own performances.
Music theory improves our sight-reading
When look at a piece of music for the first time, it is daunting to us because it is unfamiliar. We can mitigate this unfamiliarity by looking for melodic and rhythmic patterns and using what we know of the key signature and tempo, but that is only the beginning. Studying music theory to a more advanced level can also help relieve some of the difficulties in sight-reading.
If we know, for instance, that a V7 (dominant seventh) chord is usually followed by I (tonic), sometimes by IV--I (otherwise known as a plagal cadence), then we can anticipate what will come next as we play through a new piece of music. We can also use our knowledge of harmonic progressions, gained from studying music theory, to predict the development of phrases. All of this will reduce the number of things we do not know as we study new repertoire.
Furthermore, how are we to approach more complicated repertoire, such as whole sonatas and concerti? In these works, it is important to identify and understand their themes, or motifs, as they develop throughout the whole work, as well as within each individual movement. All such comprehension comes under the auspices of music theory.
Conclusion
What at first appears esoteric, like calculus in pure mathematics, becomes not only understandable but essential when it is connected with a concrete purpose. I hope this short expositive has helped you to understand why it is necessary to study music theory alongside you piano lesson, not as an expedient for progressing to ABRSM grade 6 but for related more completely to the music you wish to perform.
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