Summary

The fondamentals types of chords: Major, Minor, Diminished, Augmented and Suspended;

Chord extensions, chord inversions, chord voicing;

What is the leading tone;

Basic harmonic function of the Major scale: Tonic, Predominant and Dominant chords, how do they works;

The 7 modes of the Major scale: learn to distinguish the typical sounds of these 7 scales, it will help you breaking down songs faster (there are way more like the 7 modes of the melodic minor scale, but stick to these first);

Relative scales (ex. C major and A minor) and Parallel scales (ex. C major and C minor);

The Pentatonic scales (both major and minor);

Borrowed Chords, Secondary Dominant Chords, Chromatic Chords;

Passing tones in a melody;

What is a motif;

Key modulation;

Now some concepts that i found really useful:

Modes are interchangeable;

There is no wrong music, only uninspired (which sometimes is necessary);

Major doesn't need to be happy, Minor doesn't need to be sad;

Modes are defined by the melody, not the chords (this is purely for notation, but it's an interesting concept);