Learning English Grammar: Subject-Verb Agreement
Basic Principle: Singular subjects need singular verbs; plural subjects need plural verbs.
Incorrect: Everyone wanted to cash in their stock options.
Correct: Everyone wanted to cash in his or her stock options.
Everyone is singular; therefore, the pronouns that refer to it must be singular as well.
Similarly, verbs must agree with the subject of the sentence. (If the subject is singular, the verb must be singular; if the subject is plural, the verb must be plural.) This is known as subject-verb agreement. Do not fall into the trap of making the verb agree with the noun closest to it if that noun is not the subject.
Incorrect: A portion of our efforts are devoted to marketing.
Correct: A portion of our efforts is devoted to marketing.
Learn more about Subject-Verb Agreement:
The New Fowler's Modern English Usage by R.W. Burchfield. Clarendon Press: Oxford, England. 1996.