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Subject and Verb Agreement

The subject and verb must agree in number: both must be singular, or both must be plural. Problems occur in the present tense because one must add an -s or -es at the end of the verb when the subjects or the entity performing the action is a singular third person: he, she, it, or words for which these pronouns could substitute.
Notice the difference between singular and plural forms in the following examples:
Singular
Plural
The student sings. (He or she sings)
Your children sing. (They sing)
The bird does migrate. (It does)
Those birds do migrate. (They do)
In order to find out if your subject and verb agree, you need to be able to identify the subject of your sentence.
Here are some helpful hints that will help you to decipher where your subject is and where it is not.
Where is my subject?

1- Most likely, your verb will agree with the first noun to the left of the verb:
The Supreme Court judge decides the appropriate penalty.
Subject: judge
Verb: decides
The committee members were satisfied with the resolution.
Subject: members
Verb: were

2- Occasionally, a sentence has the subject after the verb instead of before it. This strategy is often used for poetic effect.
Over the ripples glides a small canoe.
Subject: a small canoe
Verb: glides
There was a well-known writer at the meeting.
Subject: a well-known writer
Verb: was

You will not find the subject in a modifying phrase (MP), a phrase that starts with a preposition, a gerund, or a relative pronoun and that modifies the meaning of the noun or subject under discussion.
The group of students is going on a field trip.
Subject: the group
MP: of students
Verb: is
The survey covering seven colleges reveals a growth in enrollment.
Subject: the survey
MP: covering seven colleges
Verb: reveals

The speaker whom you saw at the lecture is one of the state senators from Minnesota.
Subject: the speaker
MP: whom you saw at the lecture
Verb: is

3- If subjects are joined by and, they are considered plural.
The quarterback and the coach are having a conference.
Subject: the quarterback and the coach
Verb: are having

4- If subjects are joined by or or nor, the verb should agree with the closer subject.
Either the actors or the director is at fault.
Subjects: actors, director
Verb: is
Either the director or the actors are at fault.
Subjects: director, actors
Verb: are

5- The relative pronouns (who, whom, which, and that) are either singular or plural, depending on the words they refer to.
The sales manager is a good researcher who spends a great amount of time surfing the Web for information.
Subject: the sales manager
Verbs: is, spends
Sales managers are good researchers who spend a great amount of time surfing the Web for information.
Subject: sales managers
Verbs: are, spend

6- Indefinite pronouns (someone, somebody, each, either one, everyone, or anyone) are considered singular and need singular verbs although they convey plural meaning.
Anyone who wants to pursue higher education has to pass entrance exams.
Subject: anyone
Verbs: wants, has
Everyone on the committee is welcome to express his/her ideas.
Subject: everyone
Verb: is
A few nouns can be either plural or singular, depending on whether they mean a group or separate individuals. These words are rarely used as plurals in modern writing.

The jury is sequestered.
Subject: jury
Verb: is
The jury are having an argument.
Subject: jury
Verb: are having
7- A few subjects look plural but are really singular or vice versa.
The news of the discovery is spreading.
Subject: news
Verb: is
The mass media have publicized the facts.
Subject: mass media
Verb: have publicized
The data amaze everyone.
Subject: data
Verb: amaze
http://leo.stcloudstate.edu/grammar/subverag.html