刻苦钻研

to study diligently

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

hardworking / assiduous

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

variant of 鑽|钻[zuan4]

to drill / to bore / to get into / to make one's way into / to enter (a hole) / to thread one's way through / to study intensively / to dig into / to curry favor for personal gain

an auger / diamond

to study meticulously / to delve into

to grind / study / research