[Singular] Is/Are [Plural]?
Should the verb be is or are? I would say 'are', but a colleague says 'is'.
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The ‘Cambridge Grammar of English’ by Carter and McCarthy states that 'In informal spoken language, speakers sometimes have a choice whether to use a singular or plural verb when there is a difference in number between the subject and the complement.' That might allow your sentence to be ‘The first and most well-known example are the reserves in the United States’. However, the same source also says that in such cases ‘Normally, and in more formal styles, the subject determines the number of the verb.’ That position seems to me to be more readily defensible.
In terms of grammar, the subject "The first and most well-known example" is singular and requires singular verb agreement. If doesn't sound natural you may rewrite as