刻苦 (kè kǔ): hardworking; assiduous

In order to get into his ideal university, he studies hard every day and often reviews late into the night.

She practiced the piano diligently from a young age and eventually won a gold medal in an international competition.

The coach praised him for training hard, noting that he always practices an hour longer than others.

To master a foreign language, one must study hard and not slack off by working sporadically.