"Time is up, and somebody better do something fast."
(A comment about the government shutdown in the readers' forum - The Washington Post.)
Is it grammatical to use "do" with singular "somebody" in the cited sentence?
"better" is a casual abbreviation of "had better". With "(had) better" we use the verb infinitive, irrespective of subject, so "do" is correct. By the way, in order to determine whether something is or isn't (or should or shouldn't be) an infinitive, it can be useful to consider an analogous example with the verb "be", since the infinitive of that verb is distinctive and is not used for any other verb part.
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"better" is a casual abbreviation of "had better". With "(had) better" we use the verb infinitive, irrespective of subject, so "do" is correct.
By the way, in order to determine whether something is or isn't (or should or shouldn't be) an infinitive, it can be useful to consider an analogous example with the verb "be", since the infinitive of that verb is distinctive and is not used for
The given sentence is what is heard in colloquial speech in the US.