Basic Principle: Singular subjects need singular verbs; plural subjects need plural verbs. My brother is a nutritionist. My sisters are mathematicians.
See the section on Plurals for additional help with subject-verb agreement.
The indefinite pronouns anyone, everyone, someone, no one, nobody are always singular and, therefore, require singular verbs.
Everyone has done his or her homework.
Somebody has left her purse.
Some indefinite pronouns — such as all, some — are singular or plural depending on what they're referring to. (Is the thing referred to countable or not?) Be careful choosing a verb to accompany such pronouns.
Some of the beads are missing.
Some of the water is gone.
On the other hand, there is one indefinite pronoun, none, that can be either singular or plural; it often doesn't matter whether you use a singular or a plural verb — unless something else in the sentence determines its number. (Writers generally think of none as meaning not any and will choose a plural verb, as in "None of the engines are working," but when something else makes us regard none as meaning not one, we want a singular verb, as in "None of the food is fresh.")
None of you claims responsibility for this incident?
None of you claim responsibility for this incident?
None of the students have done their homework. (In this last example, the word their precludes the use of the singular verb.
Some indefinite pronouns are particularly troublesome Everyone and everybody (listed above, also) certainly feel like more than one person and, therefore, students are sometimes tempted to use a plural verb with them. They are always singular, though. Each is often followed by a prepositional phrase ending in a plural word (Each of the cars), thus confusing the verb...