IMO when
to grasp is used meaning
to comprehend it usually goes with an abstract noun, because if you use it to refer to an inanimate object, it might be difficult to understand whether you can’t understand the book, or you just can’t hold it firmly in your hand. So the first sentence should read something like: “I can’t grasp the meaning/essence/main idea of the book”. Also, as far as I have seen
grasp in this context is used in past tenses, passive or with modal verbs (such as
can). —
Lucky May 8 at 11:00