啃骨头

gnaw on a bone

啃光了的骨头

bone that has been picked clean

啃苹果

bite an apple

啃苹果皮

chew some apple peel

啃下一口

nibble off a mouthful

这块面包皮你啃不动。

You won't be able to chew the crust on this loaf of bread.

啃书本

pore over a book

啃不动的事

something which cannot be mastered

这事你啃不动。

You won't be able to get to grips with this.