grasp vs understand

grasp is a noun but understand is not a noun.

Word	Noun	Adjective	Verb	Adverb
grasp	Yes	No	Yes	No
understand	No	No	Yes	No

As verbs, understand is a hypernym of grasp; that is, understand is a word with a broader meaning than grasp:

grasp (noun)	understand (noun)
the act of grasping
an intellectual hold or understanding
understanding of the nature or meaning or quality or magnitude of something
the limit of capability

grasp (verb)	understand (verb)
get the meaning of something	know and comprehend the nature or meaning of
hold firmly	perceive (an idea or situation) mentally
	believe to be the case
	make sense of a language
	be understanding of