When can my coach or partner give me advice, and can I leave the court?

Under Law 16.5 of the BWF Laws of Badminton:

• You may receive advice only when the shuttle is not in play, and only until the players have taken up position to serve and receive. Once you're in position for the next serve, advice must stop.
• Play can never be delayed so you can get advice or recover your strength or wind.
• You can't leave the court during a match without the umpire's permission, except during the intervals (60 seconds when the leading score reaches 8 in each game, and 120 seconds between games).

If a coach disrupts play or distracts the opposing side, the umpire calls a let (Law 14.2.5).