In the English grammar, the subject-verb agreement is perhaps the most important ground rule that every writer should master, if not perfect. However, it also a pit into, which, many writers, especially novices, fall from time to time. By definition, the term agreement means "the state of being in conformity." In the world of grammar rules, however, agreement is understood to mean harmony in the person, number, gender, and case. A subject is in a state of conformity with its predicate, when the subject and the predicate verb agree in person (i.e., first, second, or third) and number (singular or plural).
The faults in the subject-verb grammar rules happen often because verbs, with the exception of to be, follow only one form for singular and plural subjects, as well as for three persons, unless it is third person singular present. There are usually two reasons why grammar lapses occur in the language. First, there is confusion as to the subject's number, due to the other words that come between it and the predicate verb. Then, a verb is sometimes used for a subject, not according to its grammatical form, but to its meaning. In correcting mistakes of this nature, the base or true subject needs to be extracted and determine its number.
There are instances, however, when nous, even though stripped of all its modifiers and auxiliaries, can be the source of confusion for the writer or speaker. It is good to watch out for those nouns that appear plural, but are actually singular, and should, therefore, agree with a singular verb. There is no definitive list of all these nouns; so, a dictionary should accompany any writing endeavor. These nouns often have to do with fields of discipline, such as mathematics, physics, economics, politics, and ethics. Sometimes, writers, even experienced ones, think that mumps, whereabouts, headquarters, and news are plural nouns, when they are not.
On rare occasions, certain nouns appear singular, when they are essentially plural. Nouns like data, phenomena, cacti, strata, and alumni, are the plural forms for datum, phenomenon cactus, stratum, and alumnus. On the other hand, the noun information is always singular every time it appears in a sentence.
Collective nouns also can be a source of confusion in the English grammar rules. When considered as a whole or as a unit, nouns like committee, clergy, family, herd, mob, orchestra, are taken as singular collective nouns, e.g., "The clergy called for a new meeting." The same collective nouns agree with plural verbs only when members of the group are identified or separated: "The (members of the) clergy are in disagreement with regards to the use of artificial contraceptives." It can be tricky, but sometimes, common sense is required; the latter sentence conforms more with the plural verb, because the idea behind predicate object calls for it.
At the end of the day, getting it right takes more than keeping in mind and mastering the grammar rules. It is always good to have a reference book on grammar and usage and a dictionary nearby whenever one is in doubt.