Carol ... I think we may be talking about different lessons! Which is the one you're asking about? I had a quick look through them and found this
http://www.guitarforbeginners.com/fo...part-3-4-time/
It also has finger waltz in the title. It's not it, is it?
The one you're talking about
: I'm not sure what made me choose A7/C# over C ... either way I was heading for the G/D. I always experiment with various possibilities and choose one that makes my ears perk up, as I guess this one did yours.
A7 is what I'd call a 'majorised ii' (I'll use the Aussie spelling for you!) so I guess the interesting sound here is the fact that I just played the ii. The progression goes I ii I II ... that's where the ears perk up. The critical note is also the bass note, adding to the surprise.
Sorry, Fong!