What is the cognitive reason behind the fact that we say "John is a boy," but we don't say "John is a good" or "John is boy?"
A related question is: Why do we say "This is a glass," but we don't use an "a" when asking, "What is this?"
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In English, articles indicate number. So the sentence "John is a boy," indicates 1 boy, an object that we can count. However, we can not count adjectives because they are qualitative. "John is a good" doesn't work because it is trying to count that which is uncountable.
When you say or write "John is boy", an English speaker assigns the word the properties of an adjective making it a qualitative statement of "boyness". Grammatically, it doesn't work for English.
The word this is a demonstrative along with that, these, and those. It also has the function of indicating objects that we can count. Adding an article would be redundant to a demonstrative noun.
The words a and an have at least these two uses in English: a nuanced synonym to "one" and as an indefinite article used to clarify context. To an English speaker describing John as being male, "John is a boy" means John is one example of boys; "John is the boy" means John is a particular boy previously mentioned. While you would not say "What is a this?" you would say "What is a boy?" because you are not recalling a prior mention of a boy. Instead, this is a pronoun recalling something previously indicated, perhaps nonverbally. So, "What is a this" is invokes an indefinite article and also specifies a pronoun; confusing to the listener.