Because the word police is a collective noun. Collective nouns are nouns that refer to a collection or group of multiple people, animals, or things. However, even though collective nouns refer to multiple individuals, they still usually function as singular nouns in a sentence. This is because they still are technically referring to one thing: the group as a whole. Here are some examples of collective nouns:
group – A group is a single unit that is made up of a number of individuals, whether people or things.
collection – A collection is a single unit that typically consists of many similar things organized together, e.g. paintings.
- band – A band is a single unit that consists of a number of different musicians.
Collective nouns are very similar to plural nouns. Plural nouns are nouns that refer to multiple people, places, or things, and they primarily (but not always) end in “-s,” “-es,” or “-ies.” They are derived from singular nouns, and so are truly plural in form and function. For example, the following words are all plural nouns:
Both plural nouns and collective nouns can refer to multiple things. The difference is that collective nouns refer to a group of individuals in a single unit, whereas plural nouns refer to multiple individuals. To understand the difference, consider the following sentence:
"The musicians played the song beautifully."
This sentence contains the plural noun musicians. This word lets the reader know that there are multiple musicians who played the song beautifully. Now let's look at a similar sentence:
"The orchestra played the song beautifully"
This sentence contains the collective noun orchestra. This word lets the reader know that there is a group of musicians that played the song beautifully. However, it also lets the reader know that the multiple musicians are arranged into a single group. The plural noun musicians in the first sentence does not do that.
Collective nouns usually function as singular nouns in a sentence, but they are occasionally used as plurals, too. Whether they are used in a singular or plural manner can impact which verbs and pronouns should be associated with the word. The way we determine in which manner the collective noun should be used is to consider whether the members of the collective noun are being regarded as a single, whole unit, or as multiple individuals. If they are functioning as a whole, then you use singular verb tenses and pronouns; if they are acting individually, then you use plural verb tenses and pronouns. For example, the following sentence demonstrates singular use of a collective noun:
"The offense hopes to score a touchdown on its next play."
Here, the collective noun offense refers to the members of the team’s offensive unit functioning as a whole; therefore, it acts as a singular noun in the sentence. As a result, the verb hopes and the pronoun its are also singular. Compare this to the next sentence, which demonstrates plural use of a collective noun:
"The jury eat their lunches before they deliberate."
In this sentence, the collective noun, jury, refers to the jury members acting individually. As a result, jury functions as a plural noun in the sentence. This means that the plural pronouns they and their are used, as is the plural form of the verb eat.
Certain collective nouns can only be plural, such as "police." For example:
"The police are investigating the matter." (correct)
"The police is investigating the matter." (incorrect)
However, we can make the noun countable by adding more information to the sentence. If we want to specify a single member of the police, we could say:
"A police officer is investigating the matter."
Most of the time, if you'd like to talk about a single officer of the law, you say a police officer, or just an officer:
"A police officer is here." - This is okay.
"Several officers arrived." - This is also okay.
The latter sentence is fine if it's clear from context that you mean a police officer.
In the English language, there are also many different types of collective nouns that refer to different groupings of animals. There are hundreds of different collective nouns used to describe animal group names, but here are a few common ones:
flock of birds
pack of wolves
pride of lions
What you also need to watch out for, in both American and British English, is the mistake of mixing up singular and plural subjects and verbs in the same sentence, paragraph, or piece of writing. The following sentence is grammatically incorrect because the subject and verb in the first clause (government is) are singular and the subject and verb in the following clause (they have) are plural:
"The government IS by no means environmentally perfect: they HAVE invested only £37m in renewable energy sources this year."
To ensure consistency and good English, the sentence should be rewritten either as:
1."The government IS by no means environmentally perfect: it HAS invested only £37m in renewable energy sources this year."
2."The government ARE by no means environmentally perfect: they HAVE invested only £37m in renewable energy sources this year."