please tell me what this lines mean
"He will have been eating his dinner for 15 minutes when his favourite tv programme starts."
this is uncertainity how can one say that in future he will have been eating for 15 min
exactly what is future perfect continuous?
If, for instance, he needs 30min to eat his dinner (pancakes), he starts eating at 20:00 and his fav programme (the Wheel of Fortune) starts at 20:15, you can make such a statement. This construct enables you to refer to the past... in the future.