I encountered a question that went like this:
_______ fun it is to watch a 4D movie! A. What B. What a C. How D. How a
The key to this question is A.
I've read an explanation which says " ‘How’ can only precede an adjective in this kind of sentence, e.g. “How happy I am,” or an adverb in a similar kind of sentence, e.g. “How well you dance.” Since ‘fun’ is a noun, only “What fun it is” is correct. It’s the same construction as “What nonsense you speak” and “What an awful road this is.” However, “How much fun it is” would be correct.
Of course, some people might say “How fun it is,” but it’s still grammatically incorrect."
I was confused, because "fun" is also an adjective word. Then why is "how fun" grammatically incorrect?