Being able to find the right subject and verb will help you correct errors of subject-verb agreement.
Basic Rule. A singular subject (she, Bill, car) takes a singular verb (is, goes, shines), whereas a plural subject takes a plural verb.
Example: The list of items is/are on the desk.
If you know that list is the subject, then you will choose is for the verb.
Rule 1. A subject will come before a phrase beginning with of. This is a key rule for understanding subjects. The word of is the culprit in many, perhaps most, subject-verb mistakes.
Hasty writers, speakers, readers, and listeners might miss the all-too-common mistake in the following sentence:
Incorrect: A bouquet of yellow roses lend color and fragrance to the room.
Correct: A bouquet of yellow roses lends . . . (bouquet lends, not roses lend)
Rule 2. Two singular subjects connected by or, either/or, or neither/nor require a singular verb.
Examples:
My aunt or my uncle is arriving by train today.
Neither Juan nor Carmen is available.
Either Kiana or Casey is helping today with stage decorations.
Rule 3. The verb in an or, either/or, or neither/nor sentence agrees with the noun or pronoun closest to it.
Examples:
Neither the plates nor the serving bowl goes on that shelf.
Neither the serving bowl nor the plates go on that shelf.
This rule can lead to bumps in the road. For example, if I is one of two (or more) subjects, it could lead to this odd sentence:
Awkward: Neither she, my friends, nor I am going to the festival.
If possible, it’s best to reword such grammatically correct but awkward sentences.
Better:
Neither she, I, nor my friends are going to the festival.
OR
She, my friends, and I are not going to the festival.
Rule 4. As a general rule, use a plural verb with two or more subjects when they are connected by and.
Example: A car and a bike are my means of transportation.
But note these exceptions:
Exceptions:
Breaking and entering is against the law.
The bed and breakfast was charming.
In those sentences, breaking and entering and bed and breakfast are compound nouns.
Rule 5. Sometimes the subject is separated from the verb by such words as along with, as well as, besides, not, etc. These words and phrases are not part of the subject. Ignore them and use a singular verb when the subject is singular.
Examples:
The politician, along with the newsmen, is expected shortly.
Excitement, as well as nervousness, is the cause of her shaking.
Rule 6. With words that indicate portions—a lot, a majority, some, all, etc.—Rule 1 given earlier is reversed, and we are guided by the noun after of. If the noun after of is singular, use a singular verb. If it is plural, use a plural verb.
Examples:
A lot of the pie has disappeared.
A lot of the pies have disappeared.
A third of the city is unemployed.
A third of the people are unemployed.
All of the pie is gone.
All of the pies are gone.
Some of the pie is missing.
Some of the pies are missing.
NOTE
In recent years, the SAT testing service has considered none to be strictly singular. However, according to Merriam-Webster’s Dictionary of English Usage: “Clearly none has been both singular and plural since Old English and still is. The notion that it is singular only is a myth of unknown origin that appears to have arisen in the 19th century. If in context it seems like a singular to you, use a singular verb; if it seems like a plural, use a plural verb. Both are acceptable beyond serious criticism.” When none is clearly intended to mean “not one,” it is followed by a singular verb.
Rule 7. In sentences beginning with here or there, the true subject follows the verb.
Examples:
There are four hurdles to jump.
There is a high hurdle to jump.
Here are the keys.
NOTE:
The word there’s, a contraction of there is, leads to bad habits in informal sentences like There’s a lot of people here today, because it’s easier to say “there’s” than “there are.” Take care never to use there’s with a plural subject.
Rule 8. Use a singular verb with distances, periods of time, sums of money, etc., when considered as a unit.
Examples:
Three miles is too far to walk.
Five years is the maximum sentence for that offense.
Ten dollars is a high price to pay.
BUT
Ten dollars (i.e., dollar bills) were scattered on the floor.
Rule 9. Some collective nouns, such as family, couple, staff, audience, etc., may take either a singular or a plural verb, depending on their use in the sentence.
Examples:
The staff is in a meeting.
Staff is acting as a unit.
The couple disagree about disciplining their child.
The couple refers to two people who are acting as individuals.
NOTE
Anyone who uses a plural verb with a collective noun must take care to be accurate—and also consistent. It must not be done carelessly. The following is the sort of flawed sentence one sees and hears a lot these days:
The staff is deciding how they want to vote.
Careful speakers and writers would avoid assigning the singular is and the plural they to staff in the same sentence.
Consistent: The staff are deciding how they want to vote.
Rewriting such sentences is recommended whenever possible. The preceding sentence would read even better as:
The staff members are deciding how they want to vote.
Rule 10. The word were replaces was in sentences that express a wish or are contrary to fact:
Example: If Joe were here, you’d be sorry.
Shouldn’t Joe be followed by was, not were, given that Joe is singular? But Joe isn’t actually here, so we say were, not was. The sentence demonstrates the subjunctive mood, which is used to express things that are hypothetical, wishful, imaginary, or factually contradictory. The subjunctive mood pairs singular subjects with what we usually think of as plural verbs.
Examples:
I wish it were Friday.
She requested that he raise his hand.
In the first example, a wishful statement, not a fact, is being expressed; therefore, were, which we usually think of as a plural verb, is used with the singular subject I.
Normally, he raise would sound terrible to us. However, in the second example, where a request is being expressed, the subjunctive mood is correct.
Note: The subjunctive mood is losing ground in spoken English but should still be used in formal speech and writing.
Source: http://www.grammarbook.com/grammar/subjectVerbAgree.asp
