subject-verb agreement
The verb takes the number of the subject.
The verb agrees with the subject, not with a noun that stands nearer to it. A prepositional phrase after the subject does not change the number: The box of files is ready. With or or nor, the verb agrees with the nearer subject: Neither the clerk nor the files are ready.
Examples
UseThe box of files is ready.
UseNeither the clerk nor the files are ready.
AvoidThe box of files are ready.