An automatic watch stops when you do not wear it enough: it has no battery, it winds itself from the movement of your wrist. This is neither a fault nor a defect. Here is how to start it, set it and keep it on time.
A new watch almost always arrives stopped: it has spent several days motionless in its box. Before wearing it, wind it by hand. Unscrew the crown if your model has a screw-down one, then turn it forwards, in the neutral (unpulled) position, about thirty turns. The watch starts. You can also shake it gently from side to side for a minute, but hand winding is more effective and gentler on the mechanism.
The crown generally has three positions.
The rule not to forget: never change the date between 9 pm and 3 am. During that window the date-change mechanism is engaged, and forcing it can break a part. To set it cleanly: pull the crown fully out, advance the hands until the date changes (you then know it is midnight), then set the hands to the real time, taking morning or afternoon into account.
If your crown is a screw-down type, do not forget to screw it back down fully afterwards: it is what keeps the watch water resistant.
This is how long the watch runs once fully wound: around 40 hours on most Japanese automatic movements. In practice, if you wear it all day and put it down in the evening, it will still be running the next morning. If you leave it in a drawer over a whole weekend, it will have stopped by Monday. Nothing serious: you wind it and set it again.
Yes, within limits. A mechanical watch does not have the regularity of a quartz. A deviation of a few seconds a day is normal operation for a series automatic movement. What affects accuracy:
A watch winder is the box that keeps the watch turning continuously. It is useful if you own a watch with a complicated calendar that is tedious to reset. For a three-hand watch with a date it serves no purpose, and some watchmakers even consider that it wears the movement for nothing. Winding by hand for thirty seconds in the morning does the job perfectly.
No. In the vast majority of cases it has simply run out of power reserve. Wind it about thirty turns of the crown and it starts again.
About eight hours of normal activity is enough to keep it wound. If you work seated without moving your arms, a manual top-up in the morning is a good habit.
No. Automatic movements have a clutch that slips once the mainspring is full. You cannot break anything by winding.
To understand what happens inside, read the Seiko NH35 movement explained, and for long-term care, our maintenance guide.