The basic subject-verb agreement rule in English is very simple. It states that a singular subject takes a singular verb, while a plural subject takes a plural verb. However, there are a few problems with this formulation of the rule that need to be mentioned.
To begin with, the rule makes it sound as if each and every verb has one singular form that is used with all singular subjects and one plural form that is used with all plural subjects. This is not true. If we disregard the verb be and the modal auxiliaries, all verbs have one form that is used in the third person singular, that is, with the pronouns he, she, and it, and with subjects that could be replaced by one of these three pronouns, as in example (1) below, and one form that is used with all other subjects, i.e. first and second person singular subjects (2) and all types of plural subjects (3):
(1) My sister has a baby.
(2) I have a headache and you have one too.
(3) They know her well.
The rule also makes it sound as if plural agreement is of importance in all tenses. This is not true either. Except for the case of the verb be, subject-verb agreement only takes place in the present tense. So, what we really need to remember, if we simplify the situation somewhat, is to put an -s on the verb in the third person singular (and to use the correct forms of be, have, do, and verbs like try and deny, which become tries and denies in the third person singular).
However, one problem remains. How do we know in each and every case whether the subject is (third person) singular or plural? In most cases, this is not a problem, since if the subject is a single person, animal, or thing, we have singular agreement, and if the subject is more than one person, animal, or thing, we have plural agreement.
In other words, as pointed out above, if he, she, or it could be used instead of the subject, we have (third person) singular agreement, but if we could use they instead of the subject, we have plural agreement. This is what is illustrated in the box below.
In the examples in the box, as well as in the examples used to illustrate the rules below, the relevant subjects appear within square brackets, while the heads of the relevant subject noun phrases and the first verb (i.e. the agreeing verb) of the verb phrase appear in boldface.
[She/He/it] talks. = Singular subject and singular verb
The pronouns she, he, and it are examples of third person singular subjects, and the -s on talks indicates that talks is a third person singular verb.
[They] talk. = Plural subject and plural verb
No -s on the verb, since the subject they is plural.
[The kid] talks. = Singular subject and singular verb
The subject the kid is third person singular, since the head of the noun phrase functioning as the subject is the third person singular noun kid. Therefore we use the third person singular verb form talks.
[The teachers] talk. = Plural subject and plural verb
No -s on the verb, since the head of the noun phrase functioning as the subject is the plural noun teachers.
However, there are several cases where the facts are more complicated than this. Otherwise, subject-verb agreement would not be such a big issue for people writing in English. Some of the more important of those more complicated cases will now be listed and exemplified, and, in some cases, briefly discussed.
Before we turn to this discussion it must be stated very clearly that when we say that the subject and the verb must agree with each other, we mean - in the case of noun phrase subjects - that the head word of the noun phrase must agree with the first verb of the verb phrase.
Singular noun phrases connected by or
Two singular noun phrases connected by or which together function as the subject require a singular verb:
(4) [My auntie or my uncle] is arriving by train today.
Please note that it is always the first verb (is in this case) in the verb phrase functioning as predicate verb (is arriving in this case) that agrees with the subject.
Also note that in the English examples that are used to illustrate these rules about subject-verb agreement, as well as in the examples in the box above, the relevant subjects appear within square brackets, while the heads of the relevant subject noun phrases and the first verb (i.e. the agreeing verb) of the verb phrase appear in boldface.
Finally note that since my auntie and my uncle is a conjoined noun phrase, we have two noun phrase heads, namely auntie and uncle.
Singular noun phrases connected by either/or
When two singular noun phrases connected by either/or or neither/nor together function as the subject, they normally take singular verb agreement in formal English:
(5) [Neither Jan nor Anna] is (are) available.
(6) [Either James or Carla] is (are) helping today with the presentations.
The two singular noun phrases in each example are highlighted (boldface), and so is the singular verb. The corresponding plural verb appears within parentheses, to indicate that this is an alternative in less formal types of writing and speaking.
Connected singular and plural noun phrases
When a singular and a plural subject are connected by either/or, neither/nor, or or, put the plural subject last and use a plural verb:
(7) [Neither Annika nor the others] are available.
(8) [Either Annika or the others] are available.
(9) [Annika or the others] are available.
(10) [The serving bowl or the plates] go in that cupboard.
In a case like this, that is, when we have two conjoined noun phrases functioning as the subject, we want the verb to agree with the closest noun phrase head. Since it feels awkward to have singular agreement when one of the noun phrases that make up the subject is plural, it is a good idea to put the plural noun phrase closest to the verb and have plural agreement.
Noun phrases conjoined by and
As a general rule, use a plural verb with two or more noun phrases that together constitute the subject when they are connected by and:
(11) [A car and a bike] are my means of transportation.
Subjects containing along with, as well as, and besides
When (the head word of) the subject is separated from the verb by expressions starting with words such as along with, as well as, and besides, ignore these expressions when determining whether to use a singular or plural verb:
(12) [The politician, along with the journalist,] is expected tomorrow.
(13) [Excitement, as well as nervousness,] is the cause of her stutter.
In other words, along with, as well as, and besides do not behave in the same way as the conjunction and when it comes to subject-verb agreement (even though they have roughly the same meaning or function).
Indefinite pronouns and agreement
The pronouns each, everyone, every one, everybody, anyone, anybody, someone, and somebody are singular and require singular verbs.
(14) [Each of the contestants] sings well.
(15) [Every one of the experiments] is complete.
(16) It seems as if [everybody] is mistaken.
Everyone is one word when everybody could be used instead. The expression every one consists of two words when the meaning is each one. Neither everyone nor everybody can be followed by an of phrase (so we have to say every one of the experiments, instead of *everyone of the experiments).
Sums of money and periods of time
Use a singular verb with sums of money or periods of time, that is, do not let the verb agree with the head word of the noun phrase subject, but rather with the singular sum of money or period of time:
(17) [Ten dollars] is a high price to pay.
(18) [Five years] is the maximum sentence for that offence.
Even though both dollars and years are plural, we get singular agreement, since we are dealing with one (singular) sum of money in the first example, and one (singular) period of time in the second example.
Words that indicate portions
With words that indicate portions, e.g. percent, fraction, part, majority, some, all, none, remainder, and so forth, look at the noun in the of phrase (the complement of the preposition) to determine whether to use a singular or a plural verb. If the complement of the preposition is singular, use a singular verb. If the complement of the preposition is plural, use a plural verb:
(19) [Fifty percent of the pie] has disappeared.
The singular pie is the head word of the NP (the pie) functioning as the complement of the preposition of.
(20) [Fifty percent of the pies] have disappeared.
The verb agrees with the plural pies, i.e. the head of the NP functioning as the complement of the preposition of.
(21) [One-third of the city] is unemployed.
(22) [One-third of the people] are unemployed.
(23) [All of the pie] is gone.
(24) [All of the pies] are gone.
(25) [Some of the pie] is missing.
(26) [Some of the pies] are missing.
(27) [None of the garbage] was picked up.
(28) [None of the sentences] were punctuated correctly.
(29) [Of all her books, none] have sold as well as the first one.
Please note that the fact that we can say both
(30) [Some of the pie] is missing.
(31) [Some of the pies] are missing.
could be taken to show that pie has both an uncountable and a countable use. Garbage, on the other hand, can only be used as an uncountable, i.e. we cannot say (32), or anything like that.
(32) *[Some of the garbages] were picked up.
Example (29) above, repeated here as (33), shows that the verb agrees with the head of the noun phrase functioning as the complement of the preposition of in these cases, even when the complement of the preposition has been fronted (i.e. when it does not follow the preposition to which it belongs, but appears at the very beginning of the clause or sentence):
(33) [Of all her books, none] have sold as well as the first one.
Uncountable nouns are always treated as singular when it comes to subject-verb agreement:
(34) [This wine] is not as sweet as that we were offered last Christmas.
(35) [Gravity] is an important force.
(36) [This information] is useless.
(37) [Research] tends to take a lot of time.
Dependent clauses and agreement
Dependent clauses functioning as subjects are treated as singular:
(38) [That Paul might be a thief] has never occurred to Mary.
(39) [What he failed to understand] was how she managed to escape.
If you were to argue that it is actually how she managed to escape which is the subject, you may be right, but it would not change the fact that clauses take singular agreement, since how she managed to escape is also a clause.
When two dependent clauses, for instance two non-finite dependent clauses with present participles as predicate verbs, are conjoined and together constitute the subject, we get plural agreement, as in the following example:
(40) [[Listening to music] and [watching movies]] are my favourite pastimes.
Agreement with the right noun phrase
The first verb of a verb phrase functioning as predicate verb does not necessarily agree with the head of the closest noun phrase, but with the head of the noun phrase functioning as subject in the clause in which the verb phrase in question functions as predicate verb:
(41) I know that [my mother, who has four siblings,] loves me.
(42) We need to understand that [native speakers of English] get subject-verb agreement right more or less automatically.
This rule sounds rather complicated, but it is not. The rule in itself is an example of what it might look like when we practice what we preach, in the sense that we make our sentences as clear, explicit, and unambiguous as possible. This means that anyone who knows the meaning of the words used in rule 11 also knows the exact meaning of it.
As the examples above show, there can be noun phrases between the predicate verb and the head of the noun phrase functioning as subject. In the first sentence (41), the noun phrase subject contains the relative clause
(43) who has four siblings
The last constituent of the relative clause is the noun phrase four siblings. This noun phrase is obviously plural, but since the verb agrees with the head of the noun phrase functioning as subject, it does not agree with the plural siblings, but instead with the singular mother.
The second example (42) illustrates the same fact. The only difference is that the head of the subject noun phrase is now plural (people), while the head of the NP closest to the predicate verb, i.e. the complement in the prepositional phrase functioning as postmodifier to the head people, is singular (English).
Moreover, it is important to understand that one and the same sentence may consist of more than one clause. If there is more than one clause in a sentence, there will be more than one predicate verb. Each predicate verb must agree with the subject of the clause to which it belongs, if there is a subject in the clause.
Please note that a non-finite clause need not contain a subject. If we have a look at our first example sentence above, we may conclude that it consists of three clauses, since it contains three predicate verbs, namely know, has, and loves.
These three verbs happen to be finite, so the clauses in which these verb phrases function as predicate verbs must also be finite. This means that there must also be subjects with which the predicate verbs must agree. The predicate verb know agrees with the subject I, the predicate verb has agrees with the subject who (which is coreferential with my mother, and thus third person singular) and the predicate verb loves agrees with the subject my mother, who has four siblings, which is third person singular.
If we want to understand all this, we need to know about clause elements, clauses, and phrases (and their internal structure). If you feel like reading up on this, please follow the links below.
Some important exceptions and words of advice
There are some additional facts that we need to be aware of and pay attention to. To begin with, there are a number of nouns whose plural forms do not include a plural -s. Particularly important examples for people writing academic prose include the following:
(44) phenomenon - phenomena, criterion - criteria, and formula - formulae
Relevant to mention in this context is also that there are nouns where both the singular and the plural form end in -s, such as the following:
(45) hypothesis - hypotheses, analysis - analyses, thesis - theses, parenthesis - parentheses
What all these words have in common when it comes to subject-verb agreement, is that the singular form takes singular agreement and the plural form takes plural agreement, regardless of whether the particular form in question happens to end in an -s or not.
The same goes for nouns that look the same in the singular and in the plural:
(46) sheep - sheep, hovercraft - hovercraft
and nouns that end in an -s, but happen to be uncountable and thus singular:
(47) news, aerobics, diabetes, and statistics (the subject).
Another fact that we need to pay attention to is that it is not always the case that we get plural agreement when two singular noun phrases are conjoined. If the two nouns are seen as forming a unit of some sort, normal plural agreement does not occur:
(48) [Bangers and mash] is my favourite dish.
(49) I will see to it that [law and order] prevails.
(50) [Egg and bacon] costs more than fried chicken nowadays.
In conclusion, in addition to knowing the rules stated above, you sometimes need a good dictionary to find out whether a certain noun that you want to use is countable or uncountable, and if it is countable, if it is regular or not, in order to get the agreement between the subject and the verb right.
You also need to understand that exceptional things may happen when noun phrases are conjoined. Sometimes the conjoined noun phrases are seen as referring to a unit, in which case we get singular agreement, but if the two noun phrases actually are seen as referring to two separate entities/substances of some sort, we get plural agreement (regardless of whether the nouns as such are countable or uncountable). The following two examples are intended to illustrate this last point:
(51) [Gin and tonic] is my favourite drink
(52) [Gin and tonic] are the two main ingredients in a gin and tonic.