weak vs grammar

weak is not a noun while grammar is a noun.

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

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

weak (adjective)	grammar (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