坚硬 (jiān yìng): ยากมาก

This stone is extremely hard; an ordinary hammer can't break it at all.

In the cold winter, the ice on the lake becomes very hard, and people can skate on it.

He has a tough personality; when facing setbacks, he never easily gives in and always sticks to his principles.

The surface of this new material is even harder than steel, so it is widely used to make bulletproof glass.