Words like some, part, all, none, half and other fractions need a singular or plural verb. This depends on the words that follow “of the.”
Some of the students are not here.
- Since “students” is plural, the verb is also plural (are).
A third of the employees are leaving early.
- In this case, “employees” is plural, so the verb is also in plural.
All of the milk was bought at the store.
- “Milk” is singular, so the verb is also singular.
“A number” is a special case, but it is usually equivalent to “several” or “many.”
A number of medical cases were considered easy to cure.