rules for subject - verb agreement

1. A verb must agree in number with its subject, whether the subject precedes or follows the verb.
Examples:
The students are sitting outside the building.
There are three students sitting outside the building.

2. A singular linking verb is used with a singular subject.
Example:
Bonzo the clown is here.

3. A plural action verb is used with a plural subject.
Example:
Dr. Watson and Dr. Crick work in this laboratory.

4. A singular action verb is used with a singular subject.
Example:
The president signs bills into laws.

5. Two subjects joined with “and” become plural and require a plural verb.
Example:
Joe and Dan walk to school.

6. Two subjects joined with “or” or “nor” continue to require a singular verb.
Example:
Either Jenny or Sue wins every game.

7. When both singular and plural subjects are joined with “or” or “nor”, the verb must agree in number with the subject nearest to it.
Example:
Neither a steak nor French fries appeal to me.

8. When the modifying words “each”, “any”, or “every” precede the subject or subjects, the whole unit is considered singular and requires a singular verb.
Example:
Every man, woman, and child has a birthday.

9. There are five indefinite pronouns (“some”, “part”, “half”, “most” and “all”) that have no clear number, so the noun in the prepositional phrase following the subject determines whether the subject will require a singular or plural verb.
Examples:
Some of the meat is raw.
Some of the students are late for class.

10. A prepositional phrase following a subject usually has no effect on the number of either subject or verb.
Example: Each of the students works after school.



Source: http://www.asu.edu/duas/wcenter/subject.html