Interpreting the lines, spaces and ledger lines of the four most common clefs in standard Western music notation (treble, bass, tenor and alto clef) is the basis of music theory.
Major and minor scale theory is not only a matter of simple memorization, but it is also important in terms of the function of leading tone, tonic and dominant functions.
Understanding the function of each scale degree also pertains to notes that do not occur in a scalar fashion (i.e., leaps between notes that do not appear in the order of the scale). "Voice leading" refers to the guidelines in which common practice Western harmony approaches these leaps in four-part harmony.
Basic chord theory of tonic-functioning chords should be limited at this point to I, i and the first inversion of these two chords.
Pre-dominant chord theory should be limited (in major and minor modes) to IV (and iv), vi (and VI) and ii fully diminished.
Dominant chord function should refer to V and second inversion I and i 6/4 chords. Advanced theory should also include chromatic applied and seventh chords.