"grave lesson" is correct and usable in written English
You can use it to refer to a lesson that is difficult to learn or has severe consequences if not followed. Example: We learned a grave lesson from the economic recession: don't be too quick to spend money you don't have.
What grave lesson are they teaching to the other students looking on?
At the time, North Korea said that Qaddafi's fall was "a grave lesson" that persuading other nations to give up weapons was "an invasion tactic".
I saw The Insurance Man back in 1986 and understood it to be a grave lesson in how modernity and technocratic bureaucracy can dehumanise everyone involved, stripping us of our individuality, our dignity and our compassion.
"The Libyan crisis is teaching the international community a grave lesson," the official was quoted as saying Tuesday, proclaiming that North Korea's "songun" ideology of a powerful military was "proper in a thousand ways" and the only guarantor of peace on the Korean Peninsula.
The movie ends with a shot of the boy's father, guilt ridden and sobbing, kneeling by his son's grave, a lesson presumably learned.
But despite the best efforts of tracks like "Seksi Businessman" and "Panties" to resurrect the genre, turbofolk's body lies a-moldering in the grave, an important lesson to all of us from the mists of Serbian history: Dumb music kills.
A grave distrust of corporations — a lesson learned from the experience with the railroad builders — is explicitly encoded in state law.
The sad part is many people will go to their grave having never learned the lesson because they are not willing to be honest with themselves by exploring their own inner emotional landscape.
But their faces are grave and focused during ballet lessons in the storied Vaganova tradition, imported to Brooklyn, along with most of the teachers, from Russia.
"Based on facing the reality of this grave accident and by learning lessons from the accident, the government has decided to review the national energy strategy from scratch," a policy document said.
Johnson did so because he believed in the importance of maintaining transatlantic unity in the face of global threats a useful lesson from beyond the grave.
