na-adjective
Describes a person's attitude or will that does not give up easily; often used for persistent effort or resistance.
彼は頑強な意志で難病と闘った。
He fought the serious illness with a tenacious will.
The politician stubbornly insisted on his own opinion.
na-adjective
Describes physical toughness, durability, or robustness of objects, bodies, or systems.
この靴は頑強で、何年も履ける。
These shoes are sturdy and can be worn for years.
頑強な体を作るために毎日運動している。
I exercise every day to build a strong body.