grammar vs weak

grammar is a noun but weak is not a noun.

Word	Noun	Adjective	Verb	Adverb
grammar	Yes	No	No	No
weak	No	Yes	No	No

grammar (noun)	weak (noun)
the branch of linguistics that deals with syntax and morphology (and sometimes also deals with semantics)

grammar (adjective)	weak (adjective)
	wanting in physical strength
	overly diluted; thin and insipid
	tending downward in price
	likely to fail under stress or pressure
	lacking bodily or muscular strength or vitality
	wanting in moral strength, courage, or will; having the attributes of man as opposed to e.g. divine beings
	deficient in magnitude; barely perceptible; lacking clarity or brightness or loudness etc
	not having authority, political strength, or governing power
	(used of verbs) having standard (or regular) inflection
	deficient in intelligence or mental power
	deficient or lacking in some skill
	(used of vowels or syllables) pronounced with little or no stress