"Não é grave."

Translation: It is not serious.

[lflinares]

Although not used as much, "grave" is also an acceptable English translation

[delvi]

Is there a difference in meaning between "sério" and "grave"?

[ZuMako8_Momo]

Not really, the same difference between "serious" and "grave."

[DriesBrinkman]

What's wrong with: it is not hard

[ZuMako8_Momo]

"hard" = «duro»/«rijo»/«rígido»/«difícil»

[dusics95]

can grave mean important?

[Paulenrique]

"Grave" has a negative connotation, so I don't think it can be interchangeable with "importante".