Using the indefinite article after a demonstrative pronoun

Question

Why in the sentence

This is a great party!

use the indefinite article a rather than the definite article the? After all, the demonstrative pronoun This indicates that the conversation is about a specific party, and not about some in general.

Answer 1

This is deictic, it attracts the attention of the person to the "thing" you are talking about, in the same way you do when you point out the existence of something:

• This is a table.
• This is a house.

If the "thing" you are speaking of is the only one in its "group" then you can use the:

• This is the idea!
• This is the friend I was speaking to you about.

Try and replace this with What you can see now in front of your eyes. Would you say:

What you can see now in front of your eyes is the great party?

It makes more sense to say

What you can see now in front of your eyes is a great party.

I don't know all the languages, but I am pretty confident that this is the (!) case in many other European languages, if not in all of them.

Answer 2

John Lawler wrote:

Because the rules for article use with predicate nouns are different. A count noun like party being used as the predicate noun in a clause requires an indefinite article, no matter what its subject is:

• This is an oyster

[Contrast the non-count usage:]

• This is mud.

Article use has nothing to do with specific parties in most cases – they're just nuts and bolts, with no special meanings, but lots of slots where they fit into the grammatical machinery, usually as markers to distinguish something odd.

Answer 3

This reasoning is rushing things. "This" does not indicate that the conversation is about a party; it does not even indicate what the sentence is about: such a function is still too much for that little word; as a grammatical word (i.e. void of content, not a content word) it points out something either in the situational context (extralinguistic reference) or in some part of the discourse (anaphoric reference, if before the present point in the discourse, cataphoric reference if after); this something is characterized only partially or if characterized fully, it is assumed there is no name associated with it yet as far as the discourse acknowledges it. Once a name or a characterization has been introduced or once this something has been acknowledged as far as it is possible to identify it, "this" will be often replaced by "it"; this fact shows further that "this" is merely used to point out something that is not named or not nameable, or even something that is named but not known by some of the locutors.

• She is talking about a rave party; this is a rather new phenomenon in the world of mass entertainement. (anaphoric reference; antecedent: rave party)

• This is a great party; it will be remembered by all of us for a long time; it really has something special. (extralinguistic reference, two persons at a party and one of them referring to that particular party; antecedent: great party)

• I have to tell you this: your party is a success! (cataphoric reference; the antecedent is now "your party is a success!".)

The use of "a" is not the usual use where "a" is a referring article. This means that "a" is not used to refer to some party.

(CoGEL¹ § 5.37) Nonreferring uses of the indefinite article
The indefinite article is strongly associated with the complement function in a clause, or more generally with noun phrases in a copular relationship. Here it has a descriptive role (similar to that of predicative adjectives), rather than a referring role:

• Paganini was a great violinist.
• My daughter is training as a radiologist.
• We found Lisbon (to be) a delightful city.
• What a miserable day (it is)!

"The" does not have this property of defining something as belonging to a category.

"This is the great party." is correct but means something else; only a special context will do in order for it to correspond to something sensical. There is an indefinite number of such contexts.

Imagine friends talking together about the parties they've been to in years past; imagine further that they've kept videos of them, and that among them there is a unique one they remember as the greatest party, but as they search through the titles they have problems identifying the given party, when suddenly one of the two finds it. He/she can then say this: "This is the great party!". In this particular context, though, "the great party" is metonymically the video of it.

¹ A Comprehensive Grammar of the English Language