Simply a “wonder”ful piece of music. If they only had mixed down the awful “oooohs” and “aaahs” of the background choir to zero. By the way, the marriage between Syreeta (who Stevie Wonder wrote this for) and him lasted only a few years, so here is another essential lesson to be learned: Ladies, never believe the singer. Take the guitar player.
The first section of the intro is originally played one octave lower. With all the chord changes on the 4+beat, the tune really needs some backbeat when played solo. Therefore, the dead notes on the 2 and 4 beat are essential, otherwise it will sound kind of dull.
In search of the fourth chord: For the “off-key" filler chord in the fourth bar of the verse [A], different sources indicate different options. I got used to the Gb|Bb after a while. Alternatively (according to the Realbook) you can play an A7b9, others suggest an A7#5. All of that is queer, all of it is fine.
When the tune (in the original) modulates half step up, I step out.