To the original poster:
Your problem with the shoulder in is very common, and I’m very sure that your riding instructor will, over time, probably in 1-2 lessons, help you correct your aids so you are more effective and can get the horse to shoulder in.
The MOST important thing for YOU to do, is to listen to your instructor, and to RELAX. JUST RELAX. Don’t go running to dozens of different people asking for ‘help’ - you will get 1000 answers if you ask 1000 people, mostly just arguments on ridiculous points that are more about how individuals posting on the bb for a long time don’t like each other than anything else, is picking at how they word something rather than any real substance, and none of them can see you, your horse, or know you as well as your instructor does. You don’t need help, this is just a normal thing and a part of learning dressage. So relax! I assume you have some good reason for choosing the instructor you choose - that he or she has some experience, some skill and some ability to teach. A shoulder in is NOT a complicated, advanced movement, and instructors should know how to teach it. It is NOT a big deal.
NOW. These things take time. Don’t come running here to beg for help if something doesn’t work out instantly in your lesson. All you will get is a lot of confusion. Just do this one thing - RELAX and Give it time. Learning dressage is all about being patient with the horse, with the instructor, and most of all - YOURSELF.
To try and help you, here are some general comments and to make some sort of sense of what’s been said here, and to clarify and simplify it.
First, there are TWO parts of shoulder in. One. Angle. Two. Bend.
The angle is how much the horse’s shoulders are ‘in off the track’.
The bend is how much he is bent through his body. Hopefully, it is an EVEN bend, with the same amount of bend through the neck and most of the spine. If you looked down on the horse from above, you’d want to see an EVEN bend along his WHOLE spine, from his ears to his tail*. He would look just like a train following a smooth, slight curve on a rail road track.
That is USUALLY emphasized if you’re using too much inside rein, so there’s a big bend in the neck and none in the horse’s body. It can happen because your outside rein is too loose, so there’s nothing keeping the horse’s neck from bending too much.
For MANY people, shoulder in is the first time they find out they don’t use their outside rein enough. Especially if they ride school horses, the school horses may circle for them ‘by cue’ rather than by being ‘ridden around the circle’ with the rider using both reins correctly. The horse may simply cruise around and do a nice circle without the rider using his aids properly. When he shoulder ins, then he finds out, ‘oh gee, i wasn’t really making my circles happen’. Often, he starts doing much better circles BECAUSE he struggles for a bit with shoulder in. The shoulder in quickly improves as he starts really circling properly.
Second, the shoulder in is really just a circle. Yep. The aids are the same.
The truth however is, you don’t need to worry a single bit about how much ‘angle’ and how much ‘bend’ you have at first! Don’t make it complicated! Just see if you can bring the shoulders off the track a little bit, and don’t even worry about it!
We NEVER expect horses to burst forth instantly doing something perfectly, so don’t worry about ANY of this junk while riding! Just get a feel and try to get a little angle, a little bend, and it’s not a big deal if you come off the track a little, it just means you need to be using more inside leg, just try that next time you come down the long side.
You can also make a circle in the corner, and then use that circle to help you make the ‘bend’ and the ‘angle’ easier to get. Just do 1-2 steps, NOT a whole long side (!!!) and just get a feel for it!
Don’t pretzel yourself up or twist. If there’s no reaction to your inside leg, give a little tap with your whip behind your inside leg.
There is always a way to fix anything that comes up, and any mistake you make is NOT A BIG DEAL!!!
If you are doing circles correctly, you can do a shoulder in. You DO need to ‘add’ a little of this and a little of that. You may feel, especially at first, like you need more outside rein and inside leg than you do when doing a circle. You can almost think of the ‘inside leg keeping the horse on the track’, and the outside rein ‘moving his shoulders in off the track’.
If you are riding a lesson horse, you may be in the situation where the horse knows the shoulder in and you don’t. Getting the shoulder in out of a school horse is not very often about how subtly you breathe or meditate on your ‘center’ or how physically fit your core is, or how ‘bonded’ you are, or anything else. Very often it is just about firmly using your outside rein and being more firm using your inside leg and getting a reaction to it. REMEMBER, NO HORSE IS A CHINA DOLL, IT’S OK FOR YOU TO MOVE YOUR LEGS, USE YOUR REINS, and TRY TO GET THE HORSE TO REACT TO YOU. AND IT’S OK TO MAKE A MISTAKE! So many people come here and after the lectures they get, they are paralyzed up there in the saddle, totally confused and afraid to move a muscle! Just try something, relax, and just do what your instructor says and don’t be afraid to move or use your leg or rein!
And to be perfectly honest, it’s not that different teaching a youngster that doesn’t know shoulder in at all! It’s just not that hard. After listening to a lot of people you might think it’s akin to constructing a bionic human being or inventing a cure for cancer!
Think of it as ‘all the aids are the same’. Simplify it in your mind, clear your mind, listen to your instructor, and just try! One of the best ways to do this is to plan to sit and watch a really good trainer teach some youngsters shoulder in. It is just not a big deal, and when you watch someone do it over and over and over with a young horse, and see how NOT a big deal it is, it stops seeming like a big deal.
Third. There are TWO ways to get a ‘shoulder in on 4 tracks’. One is to have very little angle, the other is to have a very large angle. In the posts above, people got so hot under the collar they didn’t consider that.
A ‘shoulder in on four tracks’ can be made by making a very, very slight angle and bend, and can be called a ‘shoulder fore’. It is really a ‘tiny shoulder in’, and only different in that the angle and bend are less. As long as there is some bend, it is a tiny shoulder in.
A ‘shoulder in on four tracks’ due to a much bigger angle, is part of a more technical debate that you don’t even want to crack the lid on or think about at this point. This is the kind of thing readers of old dressage books love to argue about, and it is really irrelevant to the question of ‘I can’t stay on the track when I do shoulder in’. The debate is, can a horse with a huge angle to the wall, really bend the appropriate amount, and do something called a shoulder in. There are some schools of dressage that say yes, and some that say no. The OFFICIAL WORD IN THE RULES, BOTH NATIONAL AND INTERNATIONAL is NO. A shoulder in is done on a MODEST angle, with the horse’s feet making THREE tracks in the sand. Since the shoulder moves INWARD, the outside front foot and the inside hind leg make one track in the sand. THAT is what you’re going for, and THAT IS HARD ENOUGH!
*The rear part of the horse’s spine is not very flexible. There, the bend is created by how he is a little flexible in the pelvis and hind quarter joints.