Why is the sentence, "It is very fun." grammatically incorrect?

Hi howard123456789-ga,

When "very" is used in the sense of "in a high degree", it modifies an
adjective (for example "a very blue dress").

"Very" can also be used to modify a noun, but it usually has a
different meaning. For example, in the sentence "He borrowed the very
book that I just returned to the library", the word "very" means "the
same one".

Traditionally, "fun" is considered a noun. We might want to say "it is
very fun", meaning "it is fun in a high degree", but that doesn't make
sense if fun is a noun. It's like saying "it is very chair".

As pointed out in the article that pinkfreud referenced...
http://www.worldwidewords.org/qa/qa-fun1.htm
..."very" is used more and more as an adjective nowadays, and many
people would be comfortable saying "It is very fun". But to some
people it will still sound odd, so it is best to avoid saying this if
you want to sound "correct". You could, instead, say "It was great
fun" or, if you want to use "very", you could say "It was very much
fun" although this sounds quite formal.

There's more information in the dictionary entry for "very" at answers.com:
http://www.answers.com/very&r=67

In particular, see the section marked "Usage Note". Although most of
it refers to the use of "very" with past participles that have not yet
been fully-accepted as adjectives, the same principle applies when
"very" is used with nouns that have not yet been fully-accepted as
adjectives.

I hope you find this answer satisfactory. It was very fun to research.

Regards,
eiffel-ga

Many authorities have accepted "fun" as an adjective. Although your
sentence sounds rather awkward, I don't think it would be considered
grammatically incorrect if used in an informal context.

This may be of interest:

http://www.worldwidewords.org/qa/qa-fun1.htm

It is a Very light

Even www.m-w.com lists fun as an adjective (amongst others).  I don't
think the sentance is gramatically incorrect at all, at least in
American English.  I don't know what the OED has to say on the matter.

"It is very fun" sounds awkward to me, also.

I searched the NY Times archives for "very fun" and got 69 hits - the
usage seems to be almost exclusively confined to quotes, or informal,
English.

I didn't learn much from searches on other "very ..." terms done for
two newspapers - other than that the US seems to be a nation of
optimists. Here are the results pasted from an unnamed spreadsheet.
The results don't look aligned in this comment before posting, but
they may come out okay. (The results for "very poor" should be 1993
and 13 if they're still together after posting.)

	NY Times	LA Times
very fun	69	0
very enjoyable	112	1
very pleasant	808	10
very good	31542	144
very well	18086	95
very pleasing	109	0
very entertaining	190	0
very agreeable	84	1
very bad	4165	 21
very badly	830	6
very poor	1993	13
very poorly	307	1

It is more idiomatic to say, 'It is very funny'. However, this does
not mean the same thing.