"It is serious."

Translation: Det är allvarligt.

[AlexanderV206707]

Varför så allvarligt?

[Yu8]

I wonder if it is possible to use seriös instead of allvarligt

[devalanteriel]

They're mostly synonymous, although it would sound weird in some contexts. But note that it would be seriöst, not seriös, because of the det.

[ZL321]

Yeah, I'm going to report that. I wrote that but it wasn't excepted.

[ex0mo]

It's accepted now, I wrote seriös and it was accepted with a typo (seriöst)

[EttaResel]

is it allvarligt because det is used? would it be allvarlig if it was den? or wouldn't you use den in this kind of sentences?

[devalanteriel]

If you were talking explicitly about an en-word, you'd use allvarlig, correct.