comparative constructions - "a so enjoyable party" or "so enjoyable a party"

I did some exercises on http://www.examenglish.com/ and did surprisingly pretty well. At a certain stage, the sentence mentioned in the title came by. Since I would of course say "an enjoyable party", I thought the same applies to the cases in which the adverb "so" becomes inserted. Brief overview:

a very enjoyable party

{article} {adverb} {adjective} {noun}

This evidently is correct.

a so enjoyable party

{article} {adverb} {adjective} {noun}

This sentence has the same structure as the foregoing one but is, according to the website, not correct.

1. 
   

   Is "a so enjoyable party" also correct?

   
   


2. 
   
   

   Is "very enjoyable a party" therefore also correct? Does it sound good to a native speaker?

   
   


3. 
   

   What's the rationale behind this? The word classes are the same as I demonstrated above. Frankly speaking, I never read nor heard this kind of word order.

   
   

Answer

So is traditionally classified as an ‘adverb’; but this is misleading. When it is used, as in your example, to modify an adjective or another adverb, it does not stand by itself but acts as a ‘pro-adverb’ (like a pro-noun) referring to another expression which defines its current meaning. In your example, for instance, so enjoyable ‘points’ to some expression which tells us how enjoyable the party was:

The party was so^→ enjoyable ^→[that I stayed much later than I intended]. OR

[I stayed much later than I intended]^←, the party was ^←so enjoyable.

You do find this complement omitted in exclamations:

This party is so^→ enjoyable ^→[_]!

The gap implies that the party is so enjoyable that words fail you! Your hearer is invited to ‘fill in the blank’ for herself with whatever comparison seems appropriate.

Note that these uses are all predicative: they fall in the predicate, after the verb BE. So ADJ cannot be employed to modify a bare nominal: it has to modify an entire noun phrase (NP), which includes the determiner. Consequently, if you want to use the expression so ADJ as an attributive modifier, next to the NP it modifies, it must come before or after the NP:

^okIt was so enjoyable [_NP a party] that ... OR

^okIt was [_NP a party] so enjoyable that ... BUT NOT
^∗ It was [_NP a so enjoyable party] that ...

This ‘rule’ is so strong that if you want so ADJ to modify an NP which has no determiner—an indefinite plural for instance—you must put the expression after the NP.

^∗ Mrs. Vanderhooven threw so enjoyable [_NP parties] that ...

This is unacceptable; you must write:

^okMrs. Vanderhooven threw [_NP parties] so enjoyable that ...

Alternatively, you may leave the enjoyable in front of the noun, incorporating it in the NP, and employ the ‘pro-adjective’ such, which behaves with NPs very similarly to the way so works with adjectives:

^ok Mrs. Vanderhooven threw such^→ [_NP enjoyable parties] [that ...

And this is in fact the construction that will ordinarily be used for exclamations:

This is such^→ an enjoyable party ^→[_]!