very serious
Es una acusación muy grave, novato.
That's a very serious accusation, rookie.
— Tropa de élite (2007)
Estás cometiendo un error muy grave y no te das cuenta.
You're making a very serious mistake and you don't realize it.
It was a very serious mistake.
Sargento, llevo un hombre muy grave, si es que no está muerto ya.
Sergeant, I have a very seriously ill man, if he isn't already dead.
— El bueno, el feo y el malo (1966)