Originally Posted by: Julian VickersIn a 4 note chord that is constructed by taking the 1st, 3rd, 5th and 7th notes from a scale, the most important notes from that chord (the notes that make it sound like that chord) are the 3 and the 7

For example:

Eminor7: the notes are

1: E
3: G
5: B
7: D

You could take out either the E or the B from that chord and it will still sound pretty much like an Eminor7, because the 3rd and 7th are still intact.
So when it comes to substitution, all you have to do is play a chord that contains at least three of those notes (including the 3rd & 7th) such as
Gmajor7 for example (G B D F#) which is a common substitution for Eminor7

Regarding your question about single notes being substituted, I assume you mean within a chord such as:
Eminor7: E G B D
to
Eminor6: E G B C

I don't have too much to say about that one, as it's a taste thing really.