Oh Bailey and Willow certainly understand verbal cues. it's very obvious when I say to them in a stupid voice (kinda hard to describe) "do you want treats?". I say it really fast, then really slow. Bailey gets all excited and turns his head away, almost like he is shy and Willow starts wrapping herself around my legs! There is NO question that they know what I am talking about.
To get them to back, I simply say 'back, back" and then back they go.
When I ground drive Bailey I will use the words 'whoa' to stop him without pulling at the reins and he stops on a dime!
To get them to spin around (a trick we do) I simply point at their backsides and say "go round".
And, they know when it's dinner time and I call them from the back field by saying "c'mon, c'mon" (once again said in the most ridiculous voice!) They come flying up the hill.
Come to think of it, the voice that I use to talk to them must have my neighnours thinking that I'm a fruit loop!
Both horses also know their names. They can be standing together in the field and I'll call "Willow" and only she will look over.