I use "easy" with a wide range of applications; mostly with a drawn out and downward tone that means to slow down. I use it in combo with a gait to get downward transitions ("easy trot" from lope/gallop, "easy walk" from trot), or by itself in a gait to get from a trot to a jog, or a gallop to a lope, etc. I use it in my reassuring voice if they are spooking or nervous to calm them down, it's basically at that point my command for "that won't eat you." Last but not least I'll use it in trail or wherever necessary to focus them if something tricky is coming up, like a steep downhill or a step.
For forward motion I use clucks and kisses exclusively. A loud, firm, double cluck means move out, and mean it. Soft clucks mean pick up speed slightly. Kiss means canter or gallop with a leg cue for the lead.
I am still working on voice commands for turns... haha... my goal at one point was to have Gyps completely voice-only but I have not been focusing on that in a while so who knows if she'll ever do it or not :)
Edit: "Ho" means stop. Both my horses will stop instantly when you say "ho" without any rein contact so I always mean it. I don't like the idea of mixing this cue with any others just because it's kind of my last ditch safety feature-- if something happens and I need that horse to stop, I need to know that no matter how that word comes out of my mouth, they will. I'd definitely pick a different word to use to slow down.
Also, newhorsemom-- your MIL *should* know that all horses are trained slightly differently and unless she trained your horse, her commands aren't the "right" ones just because they're the ones that she uses... just a little artillery for you, but I know it can be very tough to change other people's opinions of right :)