There are certain things you should not do when the golf ball is at rest (Rule 18). There are certain things you should not do when the golf ball is in motion (Rule 19).
When the ball is at rest, it can be moved without making a stroke at it. You might do this when you’re removing some debris (loose impediments), if you lift it when not permitted, or if it moves after you have addressed it. In those cases, you must replace the ball (put it back where it was) and take a penalty of one stroke. Except (there are always exceptions):
• In searching for a ball covered by sand, in the replacement of loose impediments moved in a hazard while finding or identifying a ball, in probing for a ball lying in water in a water hazard or in searching for a ball in an obstruction or an abnormal ground condition,
• In repairing a hole plug or ball mark,
• In measuring,
• In lifting a ball under a Rule,
• In placing or replacing a ball under a Rule,
• In removing a loose impediment on the putting green,
• In removing movable obstructions.
In these cases, merely replace the ball. There is no penalty.
If a ball in play and at rest is moved by another ball in motion after a stroke, the moved ball must be replaced.
If an outside agency, such as the wind, gravity, or an animal moves the ball, it must be played where it comes to rest without penalty. One course I play on has a problem with foxes running into the fairway and stealing golf balls. Drop another ball as near as you can to where the original ball was and play on without penalty.
When a ball in motion struck by you is stopped or deflected by you or your equipment (and to be complete, your partner or your caddy) take a one-stroke penalty and play the ball as it lies.
When a ball in motion struck by you is stopped or deflected by another ball at rest, play your ball as it lies without penalty, unless both balls were on the putting green. In that case you must proceed as before, but take a two-stroke penalty.
These two items are in Rule 19
In addition, a ball struck on the putting green may not hit the flagstick, whether attended or unattended, or the person attending the flagstick. If so, take a two-stroke penalty and play the ball as it lies (Rule 17-3)
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