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.
(1) den hårde mannen 'the hard man' (2) den hårda kvinnan 'the hard woman' (3) en grön bil 'a green car' (4) två gröna bilar 'two green cars' (5) Den här frukten är god. 'this fruit is nice' (6) De här frukterna är goda. 'these fruits are nice'The fact that Swedish has no subject-verb agreement is of course one of the main reason why Swedish people often do not get subject-verb agreement right when speaking and writing in English. The other main reason is that subject-verb agreement in English is next to always a purely formal matter, in the sense that whether or not the verb agrees with the subject does not affect the interpretation of the clause in which the subject and verb in question occur. In other words, the two sentences (7) and (8) would be interpreted in the same way, even though the second one is clearly ungrammatical.
(7) He is my brother.
(8) *He are my brother.If you are looking for an exception to the claim that subject-verb agreement does not have an impact on the interpretation, try using a zero plural subject, such as sheep. However, the fact that subject-verb agreement typically has no effect on the meaning or interpretation of particular sentences and the possibility that subject-verb agreement may seem pointless from a Swedish perspective does not at all mean that we can disregard subject-verb agreement in English. Quite to the contrary, native speakers of English react strongly against subject-verb agreement errors (also known as concord errors), in much the same way as native speakers of Swedish react to erroneous sentences such as
(9) *Husen var stort.'the houses were big'where the plural noun husen does not agree with the singular adjective stort. So, even though such errors normally do not make your message difficult to comprehend, their existence tends to distract native speakers from getting the message, and if such errors occur (frequently) in your written texts, there is an obvious risk that your readers will find it hard to take your message (and you as a writer and thinker) seriously. The obvious conclusion is that subject-verb agreement errors must be avoided at all cost. However, almost all writers produce such errors sometimes, so if you should happen to produce a subject-verb agreement in one of your texts, in spite of having read and acquired all the rules mentioned here, you are definitely in good company!