The following is taken from Purdue University's English Department: http://owl.english.purdue.edu/owl/resource/599/01/
Making Subjects and Verbs Agree
This handout gives you several guidelines to help your subjects and verbs agree.
1. When the subject of a sentence is composed of two or more nouns or pronouns connected by and, use a plural verb.She and her friends are at the fair.
2. When two or more singular nouns or pronouns are connected by or or nor, use a singular verb.The book or the pen is in the drawer.
3. When a compound subject contains both a singular and a plural noun or pronoun joined by or or nor, the verb should agree with the part of the subject that is nearer the verb.The boy or his friends run every day.His friends or the boy runs every day.
4. Doesn't is a contraction of does not and should be used only with a singular subject. Don't is a contraction of do not and should be used only with a plural subject. The exception to this rule appears in the case of the first person and second person pronouns I and you. With these pronouns, the contraction don't should be used.He doesn't like it.They don't like it.
5. Do not be misled by a phrase that comes between the subject and the verb. The verb agrees with the subject, not with a noun or pronoun in the phrase.One of the boxes is openThe people who listen to that music are few.The team captain, as well as his players, is anxious.The book, including all the chapters in the first section, is boring.The woman with all the dogs walks down my street.
6. The words each, each one, either, neither, everyone, everybody, anybody, anyone, nobody, somebody, someone, and no one are singular and require a singular verb.Each of these hot dogs is juicy.Everybody knows Mr. Jones.Either is correct.
7. Nouns such as civics, mathematics, dollars, measles, and news require singular verbs.The news is on at six.
Note: the word dollars is a special case. When talking about an amount of money, it requires a singular verb, but when referring to the dollars themselves, a plural verb is required.Five dollars is a lot of money.Dollars are often used instead of rubles in Russia.
8. Nouns such as scissors, tweezers, trousers, and shears require plural verbs. (There are two parts to these things.)These scissors are dull.Those trousers are made of wool.
9. In sentences beginning with there is or there are, the subject follows the verb. Since there is not the subject, the verb agrees with what follows.There are many questions.There is a question.
10. Collective nouns are words that imply more than one person but that are considered singular and take a singular verb, such as: group, team, committee, class, and family.The team runs during practice.The committee decides how to proceed.The family has a long history.My family has never been able to agree.
In some cases, a sentence may call for the use of a plural verb when using a collective noun.The crew are preparing to dock the ship.
This sentence is referring to the individual efforts of each crew member. The Gregg Reference Manual provides excellent explanations of subject-verb agreement (section 10: 1001).
11. Expressions such as with, together with, including, accompanied by, in addition to, or as well do not change the number of the subject. If the subject is singular, the verb is too.The President, accompanied by his wife, is traveling to India.All of the books, including yours, are in that box.
SAMPLE PROBLEMS INVOLVING SUBJECT-VERB AGREEMENT TAKEN DIRECTLY FROM RELEASED SAT/PSAT'S:
(Note: I have underlined the subject and verb. Added phrases and clauses have been color-coded orange to show that they contain additional information and are NOT part of the base sentence. Always look for the base sentence to verify the agreement between the subject and verb.)
Although the conference is over, the Mexican representative, like other delegates, have declined to comment on the new trade agreement.
The results of the investigation belongs less to the realm of knowledge than to that of speculation; the writer has given us more fancy than fact.
Evidence from surveys and interviews show friendships made in high school tend to last longer than those made in college.
The scientific writings of Edward O. Wilson, Stephen Jay Gould, and Richard Dawkins, which has continued the discussion of genetic issues raised by Charles Darwin, are familiar to many high school and college students.
The governor's aides are convinced that the announcement of the investigation, coming just days before the filing deadline, were calculated to discourage the governor from running for reelection.
Air pollution caused by industrial fumes has been studied for years, but only recently has the harmful effects of noise pollution become known.
The harmfuls effects of smoking on the vascular system is increasingly well documented.
The number of awards given this year to biochemists accentuate the significant gains being made in the study of chemistry of living organisms.
Critics contend that reforms in welfare has not managed to bring the high percentage of our nation's children living in poverty the economic security that they need.
The harmful effects of smoking on the vascular system is increasingly well documented.
The main reasons students give for failing to participate in the political process is that they have demanding assignments and work at part-time jobs.