Tonic, Subdominant, Dominant are just names given to different degrees of the Major scale.

C-Major scale:
C, D, E, F, G, A, B,C (thus, all those tones are related in the previous example of related chords - C, Dm, Em, F, G, Am. The "B" which is the seventh tone of the C Major scale a diminished chord would be built on - B dim (though that's another story).

I haven't considered music theory in 30 years. I compose from the heart. Though I'm sure music theory is so brainwashed into my skull it's used subconsciously.

It's definitely not necessary to know music theory in order to compose great songs.

John smile