Two plus two is four. vs Two and two are four.

[Winwin2011]

1. Two plus two is four.
2. Two and two are four.

Sentence1 uses singular verb to be and sentence 2 uses plural verb to be. My kid asked me why? Could anybody help, please?

 

[SoothingDave]

Because the grammatical subject of the first sentence is "two," which grammatically is singular.

The subject of the second sentence is "two and two," which is two things, which makes the subject plural.

"Plus" is not a conjunction. It is a preposition. "Plus two" is a prepositional phrase acting as an adjective to modify the subject "two."