There are two critical horse training principles that appear opposite of one another.
One is getting your horse to move.
The other is getting your horse to stop.
Both of these principles are vital depending on the situation at the moment.
What's more, you need one principle to teach the other and vice versa.
In other words you must get a horse going forward before you teach him to stop.
That said, lets start with stopping.
We have to define what stopping really is.
Stopping isn't saying "Whoa" and your horse slows down.
It isn't saying "Whoa" and your horse takes a few steps and finally stops.
It means your horse should quit moving, drop dead, right here, right now and don't go forward until I say to go forward.
" Notice there is no gray area here.
It is very black and white.
In stopping there is no moving.
Stopping is stopping.
Period! Now that I've belabored that point, your horse must get that in his head too.
Start by looking at what YOU are doing with your horse.
For example, if your horse moves too fast for you and you want him to slow down, are you saying "Whoa" when you really mean slow down? If so, it's no wonder your horse won't stop.
Next, be very firm about stopping.
When you want to stop, your horse stops immediately.
Now here's a little trick to get your horse good at stopping.
Just after your horse stops, make him back up immediately after his stop.
Just one or two steps is ok.
Why do this? Because you get it in his mind that he won't be going forward.
Eventually, when you say "Whoa" he knows he's going to take a step back.
This also gets his backend under him which is important for lots of other things in the riding world.