The dominant chord is not the only chord allowed to have a seventh. Sometimes you will find predominant chords with 4 notes – a predominant seventh chord
. Adding a seventh to any predominant chord (ii⁷, iiø⁷) is possible even if they are inverted (ii6/5, or iiø6/5.) It is important to remember that a predominant seventh chord must be completely spelled. You must not omit any of the 4 notes total.
A Predominant chord (for now, built on scale degrees 2 and 4) tends to lead us to the dominant.
IV-V-I has a strong bass motion that leads into the cadence.
ii-V-I is good because the roots are part of the circle of fifths, which makes for strong motion.
ii6-V-I is even better because the root motion is still fifths, but the bass motion is 4-5-1
The seventh of any chord is considered a tendency tone. Often is called the chordal seventh. The chordal seventh must resolve down by step.
In minor keys the iv is now a minor chord, and ii7 becomes half diminished. The same rule applies to these as they do in major keys. They are still pre-dominant chords that lead to the dominant.