Most player behaviour in badminton is good, however, it's easy to forget correct behaviour when things aren't going your way. While most behaviours are obvious, there are many understandings within established clubs, good manners on and off court, that are not explicitly written anywhere but nevertheless you need to know and remember.
Most of these are NOT covered in the Laws or in the Code of Ethics and Behaviour, nevertheless are important pointers for those starting to represent their club and play more experienced players. As you read them, you will start to understand these all exist in the spirit of good manners, fairness and integrity within the sport, irrespective of age and experience.
Many players moving from the social game into the club and league environment don't know them, so here's a list.. it's not exhaustive, but hopefully it gives you an idea.
If you lose a point, send the shuttle back to the server accurately and gently. Don't smash it in the wrong direction. In a pro tournament, this would lead to a red card.
A line call is the responsibility of the receiving player, as they are the closest to the shuttle.
If you make a line call, do so with confidence, whether in or out. If you or your partner are unsure, play a let with the agreement of your opponent.
Do not question or challenge your opponent's line calls, it's usually impossible to tell and the game relies on honesty of all players. If you have a consistent problem with calling, or indeed any player behaviour (such as service legality) by an opponent, raise it with your captain, who will raise it with the opponents' captain. In a pro tournament, a direct challenge could raise a yellow card.
Be honest about a double hit, some touches are so light they might not be seen or heard by you. If another player calls out a double hit, accept it.
By all means cheer a winning point, but not if it's from an opponent's mis-hit. This is their bad luck, not necessarily your skill.
If you accidentally hit an opponent with the shuttle, immediately apologise with a raised open hand and check on the other player. Do not celebrate the point.
Don't abuse or damage any equipment on court, swear loudly or abuse your equipment - these are all red card offences. If you make mistakes, you need to train more, not have a tantrum.
If you are not ready to receive a serve, hold up your non-racket hand with an open palm. When you are ready, drop your arm. If you return the shuttle, and say "I wasn't ready" this is both impolite, and a fault.
It is the server's privilege to change a damaged shuttle. If a shuttle becomes obviously damaged during a rally, any player may call let during the rally. Non-serving layers should ask the opponent if they are happy to change the shuttle.
If a shuttle from another game lands on court, let should be called the moment it's distracting to players.
If you want to test a shuttle with a hit (to see if damage will change its flight), ask to do so before testing it.
If you change a shuttle, it's normal to hold the shuttle up so all players are aware before the first serve it's used. This is to ensure all players understand a new shuttle (which will be faster than the previous shuttle) has been introduced.
A let can be called if a shuttle lands near a sightline in the run up to a serve, even if it's not on the court.
If you're off-court, watching:
Don't move around the court when a service is about to happen, it's disrespectful to players and distracting during a moment of focus. You wouldn't like it!
It's fair and reasonable to wait to serve, or signal you are not prepared to receive a serve, while there is movement around the edge, use the time productively to talk to your partner.
As per cheering on your team, celebrate good shots and decisively won points, but let rallies carry on uninterrupted by cheers and chatter - this is fine in pro tournaments but anywhere else is considered rude and unsporting.