This is the most confusing tense because it does even make sense. They will have been happy together? Or they would have been happy together?

"They will have been happy together" does sound weird without any other context. But if you say, e.g., "When they'll get their first child next year, they will have been happy together for five years," it starts to become manageable. ;)

The first sentence is future perfect tense (indicative mood), in stark contrast to perfect tense (conditional mood) of the second sentence.

Hi Chris,

Thank you so much. Now that makes perfect sense and yes very manageable. As always to the rescue Chris, je vous remercie!

:)) you're welcome!

sorry guys.  in french perhaps it works.  this verb structure does not work in english: "will have been"  does NOT exist. only "would have been".  check out a grammar book

Hi Elaine,

Of course „will have been“ is proper English. It is called future perfect tense. 

Example: They will have been talking for an hour by the time Tom arrives.