The rule: education for the brain, not exercise for the body
A yearling's growth plates are still open and its frame is still being built, so the groundwork question divides cleanly. Anything that is really a lesson — standing, leading, yielding, waiting — is safe, because it happens at walk in short bursts and teaches the horse how to be handled for life. Anything that is really exercise in disguise — long lunging sessions, endless circles, repetitive trotting on a small rein — loads immature joints in exactly the repetitive way they handle worst. Judge every activity by which side of that line it falls on.
Leading and halting: the foundation skill
The core lesson is walking politely on a loose rope: forward when you walk, stop when you stop, stand without barging, barging being corrected by a quiet reset rather than a wrestle. Practise in straight lines and wide, sweeping turns around the yard or field rather than small circles. Add halting on a light cue and standing for a count that grows over weeks. A yearling that leads and stands beautifully is already better educated than many adult horses, and none of it cost its joints anything.
Standing for the farrier, the vet and the hose
Teach the stillness skills while there is no emergency attached to them. Stand tied or held for grooming all over, including the ticklish places. Pick up each foot daily — hold it for a few seconds, tap it gently, put it down before the yearling snatches it — so that the first real farrier visit is a non-event. Practise standing for a mock examination: hands over the body, legs run down, mouth opened. Each of these is pure education, done standing still, and each pays off for decades.
Yielding to pressure, a step at a time
A yearling can learn to move a step away from light pressure on the shoulder or hip, to back up a few paces from a gentle cue, and to lower its head when asked — the beginnings of every ridden aid, taught at a standstill or a walk. Keep the pressure light and release the instant the horse tries; with youngsters, the release is the teacher. A step or two is plenty per repetition. You are installing understanding, not drilling movement, and understanding installs best in short, successful doses.
What to leave until later
Postpone anything repetitive, circular or fast: lunging as exercise, long trotting sets, work on deep or slippery ground, and any session that runs long because the horse has not yet done what you asked. Turnout with companions is the yearling's proper exercise — free play loads the frame variably, the way it is built to be loaded. If you are ever unsure whether an activity counts as lesson or strain, ask whether it would still be fine done daily for a month; lessons pass that test, strain does not.
Follow-up questions from the yard
Groundwork questions tend to arrive standing next to the horse — is today the day to introduce the hose, how long should this session run, what comes after leading. For the ones that stall you mid-yard, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.