This book was so hard, I struggled through it for a month.
Tips
usage
啃 implies biting into something hard or tough. Figuratively, 啃书 means to study a difficult book with great effort. 啃老 means to live off one's parents (lit. 'gnaw on the elderly').
memory
The mouth radical 口 on the left shows this is about using the mouth. Think of the sound 'ken' like a crunching sound when gnawing.
