What it's the difference between rogligt and kul?
"Roligt" and "kul" are synonyms. Do note though, that "kul" is an exception adjective that never changes.
What about nöje?
To "ha kul" is, if not a phrasal verb, at least a very fixed way of expression for having fun.
Why does it tell me to add "det"? Why is it a necessary part of the sentence?
It's optional in this case.
Does Vi har kur alltid tillsammans make sense?
Har ... roligt worked. But, vi is plural, so why isn't it har roliga?
why not 'vi har alltid kul tillsammans'?
I think that makes sense. But why is the alltid and kul switched?