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Do predicate nominatives have to agree in number with the subject?
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No.

Peanut butter and jelly sandwiches are a good snack.

CJ
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It is underlined:

Peanut butter and jelly sandwiches are a good snack.
My name is Mister Micawber.
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Comments  
Is this only when treating the subject as a single entity?

Like

"Lions are the symbol of power." <<Lions treated as collective term, which makes sense.

But

"He and I are the leaders of the pack." <<He and I doesn't make sense as a single entity.
I don't know exactly what you mean by "treating as a single entity".

I was not treating -- at least I was not aware that I was treating -- peanut butter and jelly sandwichesas a single entity. Emotion: smile

I think the answer you're looking for may be "Necessarily so". The fact that the predicate nominative is singular means there must be at least one way of conceptualizing the plural subject as a single entity of some kind.

It sounds like a chicken-and-egg sort of thing.

CJ
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can you give me an examples of predicate nominative?
Emotion: left hug????????????????????????????????????????????????????
 Mister Micawber's reply was promoted to an answer.