Basic Principle: Singular subjects need singular verbs; plural subjects need plural verbs.
My brother is a nutritionist. My sisters are mathematicians.
1)The indefinite pronouns anyone, everyone, someone, no one, nobody are always singular and, therefore, require singular verbs. Examples:
Everyone has done his or her homework.
Somebody has left her purse.
None of you claims 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.
2). Prepositional phrases that come between the subject and the verb do not change the number of the subject. Some examples : as well as, in addition to, together with, along with, Examples:
The teacher, as well as the students, was working on the problem.
The mother, together with her children, is waiting.
3). If one subject is singular and one plural and the words are connected by the words or, nor, neither/nor, either/or, and not only/but also, you use the verb form of the subject that is nearest the verb.When subjects are joined by words such as neither, either, not only the verb must agree with the closer subject. Examples:
Either the man or his wife knows the answer.
Either the man or his friends know the answer.
Either the children or the man knows the answer.
Either the bears or the lion has escaped from the zoo. Neither the lion nor the bears have escaped from the zoo.
4).The conjunction or does not conjoin (as and does): when nor or or is used the subject closer to the verb determines the number of the verb. Whether the subject comes before or after the verb doesn't matter; the proximity determines the number. Examples:
Either my father or my brothers are going to sell the house.
Neither my brothers nor my father is going to sell the house.
Are either my brothers or my father responsible?
Is either my father or my brothers responsible?
5).Sometimes modifiers will get between a subject and its verb, but these modifiers must not confuse the agreement between the subject and its verb. Example:
The mayor, who has been convicted along with his four brothers on four counts of various crimes but who also seems, like a cat, to have several political lives, is finally going to jail.
6). If your sentence compounds a positive and a negative subject and one is plural, the other singular, the verb should agree with the positive subject. Examples:
The department members but not the chair have decided not to teach on Valentine's Day.
It is not the faculty members but the president who decides this issue.
It was the speaker, not his ideas, that has provoked the students to riot.