A singular subject takes a singular verb.
Nancy is the school librarian.
Plural subjects take plural verbs:
Nancy and Ned are friends.
But we don’t always write with such simple subjects. What is the correct verb form in these sentences?
A pack of wolves was/were howling in the distance.
A bundle of ballots was/were found in the official’s car.
A flock of geese is/are migrating overhead.
Each subject includes both a singular and a plural noun:
So do you use a singular or plural verb?
I could get into a lot of technical terms here like notional vs. formal agreement and predeterminer. But I prefer Merriam-Webster’s “plain sense” solution: “When you have a collecting noun phrase (a bunch of) before a plural noun (the boys), the sense will normally be plural and so should the verb.”
Another way of understanding this is to look at the parts of speech involved. A noun/subject phrase like “a bundle of ballots” includes a head noun—bundle, and a prepositional phrase—of ballots. The prepositional phrase modifies the noun, therefore the noun establishes whether the verb is singular or plural. In most cases, that will be correct.
Applying these principles to the above examples, we would choose the following verbs:
A pack of wolves was howling…
The intent or emphasis here is on the pack.
A bundle of ballots was found…
Again the sense of the subject is that many ballots were located someplace they should not have been.
A flock of geese is migrating.
Though it would not be improper to say a flock of geese are migrating, if your sense is that the emphasis should be on the geese rather than the flock.
And can’t we all agree that it’s a good thing when you get to determine what’s right and wrong?