"learn hard" is correct and usable in written English.
You can use it to motivate someone to put effort into their studies, for example, "If you want to get good grades in school, you have to learn hard."
Often, visitors learn hard truths at AgeLab: many older adults don't like products, like big-button phones, that telegraph agedness.
Now that he is no longer alone he has also had to learn hard lessons about communal living.
Employees there learn hard skills, such as project management, but also softer skills, such as politics and networking.
Rarely in the haphazard world of horse racing, where luck and injuries are sometimes more significant than talent and where optimists usually learn hard lessons, has a horse progressed through a schedule so smoothly.
As British manufacturing struggles to survive at the high end and middle-class futures look as shaky as the working-class present is uncertain, is it our turn to learn hard lessons?
The definition of easy to learn, hard to master, it essentially has just two rules governing the core play, which involves two players alternately placing black and white tiles on a 19x19 board.
The Better Move Keep kids cash-only to force them to learn hard-stop budgeting.
"I learned hard lessons as a young buck," he said.
Now in power in many countries after decades in opposition, they are learning hard lessons.
If so, it is a habit learned early and learned hard.
Yes, we are learning hard lessons.
