Author of this post is Jiří Cvrkal, thanks go his way !
I'm continuing on my post for beginners. Last time I explained what are the three basic chords in each tonal key and I hope it was not difficult to understand and so it won't be hard to understand the "secondary" chords. Those are, in contrary to the main chords, a minor ones. These chords are on II, III and VI step. When you enhance the table tool from previous post according to the picture below, you can easily find the "secondary" chords for all tonal keys.