for MD while defending you should be in a split format. You go into that whenever the bird gets cleared / lifted . You move up to the net when in the attacking mode , such as smashes . for receiving the place you go depends on your shot . if you lift off the serve, go split. if you drive it , and it puts them on defense go to the net
XD - basically never (as you are a guy and you are defeating the purpose of playing mixed - the woman is better at the net than you are, so let her go to the net). This includes when receiving serve or defending. Just don't do it.
MD - if your opponent serves short and you play an attacking reply (i.e. a net shot, a push, a drive), stay at the net. If your opponent serves high, your partner goes to the net.
When defending, if your opponent lifts the shuttle to your partner, go to the net and stay at the net until you or your partner lift. If your opponent smashes at you, play a blocked/pushed defence (not a drive) and then move forwards to the net. If you hit the block straight, then move forwards and towards the centre. If you block cross court, move directly towards where you blocked it (so if the cross it gain, you will already be close by) - a lot of people make the mistake of blocking cross and then stepping across - leaving themselves vulnerable to a cross net shot in reply.
While defending, if your opponent plays a drop shot - drive forwards with your legs, take the shuttle high, and try to kill or push the shuttle, then stay at the net. Do not play a netshot unless your opponent is not very sharp - you want to push it past the net player, not in front of them.
when u attack the short serve, should u back off or stay at net ?
It depends on how successful is your return of serve. If u are able to push the shuttle to spot that you planned (good "quality" shot), then stay at the net and get ready for the expected 3 shot that you've planned. If the return of serve is not as good as you've planned, or your opponent guessed right and ready for the push, start backing off, the fast drive is coming. U need to help out your partner; try making it a 50-50 play for the moment.
let's say my opponet smash to me, and if I return the smash to a open space, should i move up to the net?
depending on your block, or drive back. For me, when I see that i have the upper hand (a good drive), and sense that my opponent is taking the shuttle mid net height (low net height) at best, yes, by all mean, I follow my block/drive to the net. If my block "floats" (feel like forever), then stay square.