刻苦

hardworking / assiduous

quarter (hour) / moment / to carve / to engrave / to cut / oppressive / classifier for short time intervals

bitter / hardship / pain / to suffer / to bring suffering to / painstakingly

assiduous / taking great pains

to study hard / assiduous

to bear hardships and work hard (idiom); assiduous and long-suffering / hard-working and capable of overcoming adversity

to study diligently

diligent / assiduous

to study hard / assiduous