Stubborn or dogged — having a strong will or attitude and not yielding easily; persistent and resolute.
彼は頑強に反対の意志を示した。
He showed stubborn opposition.
問題に直面しても、頑強にあきらめなかった。
Even when faced with problems, he stubbornly did not give up.
頑強な性格の人は一度決めたことをやり通す傾向がある。
People with a tenacious character tend to carry through what they decide once.
Tough, sturdy, or hardy — describing physical strength, durability, or robustness.
この金属は頑強で長持ちする材料だ。
This metal is a sturdy, long-lasting material.
彼は頑強な体格で、重い荷物を難なく持ち上げた。
He has a sturdy build and lifted the heavy load with ease.