Articles

"Meek" is an adjective but it can be used as a noun, as in, "the meek shall inherit the earth. You can use "stupid" the same way – "the stupid tend to avoid using turn signals at intersections." But identifying one person as "a stupid" doesn't work like that, no more than referring to one person as "a meek" who will supposedly inherit the earth.

Any word can be used a noun if the word itself is a subject. Can "whenever" be used as a noun? Sure – say you were writing a poem and you used the word "whenever" and your teacher objected to its use. You switch the word order and ask the teacher if it's better and the teacher responds, "Yes, the whenever works in this context."

Thanks for answering my question! But I specifically wanted to know the reason why it's wrong to use the articles a/an before nouns like stupid. I just know it's wrong but I do not know the reason for it, because in some contexts, using the articles a/an before a noun alone would be correct.

Say for example I'm an alien from a different world and the ship we were riding accidentally landed on earth, and after seeing a human, one of my alien friends said: "Oh, wow! Look that's a human". I know in this context that it's correct. However, why is this rule not applicable when it comes to the noun stupid? One can't just say "Oh, hey! Look, a stupid"

I also know that it would be correct if it's a noun phrase. For example "a stupid girl", but why is it wrong to use the articles a/an before the word stupid alone.

Articles in English identify nouns as specific (the) or unspecific (a, an), or definite and indefinite. The reason one can't just say, "Oh, hey! Look, a stupid" is simply because "stupid" is not a noun. If you are trying to use "stupid" as a noun, you'd need to provide the context which explains why it's being used as a noun.

Oh, alright. That kind of cleared up my confusion. So, if i understand it correctly, stupid in the example I've given acted as an adjective and you cannot use the articles before adjectives, right? Unless they're followed by a noun. Example: "oh, hey! Look, a stupid alien"

Now, for the context of stupid being used as a noun, I am just going to use the example I've seen on google. Here it is: "You're not a coward, stupid!" If we were to rephrase it this way: "You're not a stupid, but you're a coward." Would it still be correcr? Here, I also used it as a noun, but why is it wrong to use the article 'a' before it when it's already acting as a noun?

In this case "stupid" takes the place of a proper noun.

"You're not a coward, stupid!"
"You're not a coward, John!"

But this just isn't an English sentence. "Stupid" is still an adjective:

"You're not a stupid, but you're a coward."
"You're not a stupid person, but you're a coward."

In order to use a nominal adjective like the stupid, the poor, or the meek you must be referring to a plural subject and it must take the definite article.

Thank you so much!