In my ell answer, version 32, I provided the following, problematic, wording (especially bold italic), and I need help to better understand this issue so I can fix my answer:1
The thing is the books. (Reduced form of Sentence 2.a)
Fundamentally, Sentence 2.a (the so-called "correct" answer), is grammatically defective. Recall there is another grammatical rule: the subject and subject's complement should match in number.
A clarification is needed here. I am not suggesting here, in this EL&U question, that there is any such singular grammatical rule as indicated in the quote above. The quote continues as follows:
The reduced sentence makes the disagreement between the subject's and complement's plurality obvious. What is one to do? Language is linear and we know "The thing is X" is better than "The thing are X", so we go with the former, which is the subject-verb agreement rule. But what is "the books" then? It must be thought of as a collective noun, even if it doesn't look or feel like one. Forcing something to be a collective noun is related to the idea of notional agreement.
Others noted that grammatically defective is too strongly worded. There is no such "grammatical rule" for subject/complement agreement. Please help me understand/improve upon my line of thought here. I'm stating that Sentence 2.1 presents a fundamental "conflict" or "error" (however one might define error) at some level (morphological, syntax, grammatical, semantic):
The thing is the books. (Or, "The thing is the objects.")
Based on the "obvious" morphosyntactic plurality conflict, the above sentence feels so wrong that I think many native speakers would think it "simply" grammatically incorrect. However, @F.E. gives counter examples:
And here are some more grammatical examples, where the number of subject and predicative complement don't agree: "They were a problem to us all", "That so-called work of art is simply four pieces of driftwood glued together" (CGEL, page 254-5). As CGEL says: "What is required is semantic compatibility, not syntactic agreement . . .". There are more examples on page 512: "The only thing we need now is some new curtains", "The major asset of the team is its world-class opening bowlers", "Our neighbors are a nuisance", "This gadget is five different tools in one".
I think all the examples given above can be explained as notional agreement.
Question 1: If we explain this phenomena in terms of notional vs syntactic agreement, where does "grammatically correct/incorrect" fit in?
Question 2: Can a simple sentence such as "The thing is the objects." be grammatically incorrect while more complex sentences such as "The thing is four pieces of driftwood glued together." be grammatically correct? (The answer, in my mind, must be no; this is my conundrum.)
Question 3: Would it be better to say that Sentence 2.a has a low level of (linguistic) grammaticality?
Question 5: Do we simply draw a hard line and say "The thing is the objects." is 100% grammatically correct? I didn't particularly like this option based on intuitive notions of "grammatically correct".
1. Please help me and be kind; I'm trying to improve my understanding. I'm looking for cool, objective advice on how to look at this.
I numbered my questions so they can be easily referenced, if desired. My preference is the final answer would sufficiently answer all the questions, but of course that can be done explicitly or implicitly.