Music theory is mind blowingly interesting and at the higher levels quite difficult. I was doing modulation today. Its a great skill to have and enables you to work out how to get to different keys by using the ii, V, or IV, V chord patterns.
I was also practising writing out chords and recognising modes from chord patterns by the chords, the dominant 7 and the half diminshed seventh which are the key - so to speak - to working it out.
I have recently been working out the keys of a folk song which modulates from B minor, to C minor, to D minor and then to F# minor. Its good practice.