X: They are too fat/beautiful(adj) women(n). (wrong)
This food contains too much(adj) salt(n).
How do you say #1 sentence is wrong while #2 is correct?
Could you please explain the nuance of the grammar?
Thanks.
Adjective phrases, as used in your first example, when introduced by degree modifiers like "too", "as" etc. can only function before the article "a", and are thus restricted to singular noun phrases:
"She is too fat/beautiful a woman". NOT *"She is too fat/beautiful woman". NOT *"They are too fat/beautiful women".
"It's as fine a show as I've seen". NOT *"It's as fine show as I've seen". NOT *"They're as fine shows as I've seen".
Your other example is a different construction. "Much" is not an adjective here, but a determiner which has "too" as a modifier. Note that it is restricted to non-count noun phrases: "Salt" is non-count in your example, and hence okay, as is non-count "trouble". But "vegetables" and "problems" for example are count nouns here, and hence ungrammatical:
"This food contains too much salt". NOT *"This food contains too much vegetables".
"It was too much trouble". NOT *"It was too much problems.