Although not used as much, "grave" is also an acceptable English translation
Is there a difference in meaning between "sério" and "grave"?
Not really, the same difference between "serious" and "grave."
What's wrong with: it is not hard
"hard" = «duro»/«rijo»/«rígido»/«difícil»
can grave mean important?
"Grave" has a negative connotation, so I don't think it can be interchangeable with "importante".