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I'm trying to figure out how to word the following hyperbolic statement regarding dodgy building construction:
The only sturdy thing about that building [is/are] the nails holding it together.
Which is correct in this case, is or are? Neither sounds correct.
Even if I reword the sentence, it still sounds awkward:
The nails holding that building together [is/are] the only sturdy thing about it.
We have three subjects; two singulars and a plural. Grammatically, the main subject is the "sturdy thing", but the nails are the punchline (the context is supposed to be funny).
What's the rule here?