When you are caught off guard at the forehand corner at the backcourt, buy you can defend using the forehand defense.
There is two places where you can hit the shuttle which is when the shuttle is under your shoulder or parallel to your shoulder.
There is two locations where you can send the shuttle. One is clear to the back of the court or just hit the shuttle back to the net.
Your footwork when doing this shot is you must move backwards using your right foot. After that, cross your legs across and bend your knees like a bow in a diagonal position. Most importantly, you must lock your elbow then only hit the shuttle. Another point to take note is that your elbow must be facing forward.
Watch this video and learn more on how to make this shot.