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1) If the word police is plural, then we have such a sentence

Police are here

Okay, no questions, but what to do with the tag question?

Police are here, aren't it?

or

Police are here, aren't they?

Because I think it wouldn't be

Police are here, aren't police?

We should change it for some pronoun

2) We can have some dialogue:

A: Do police work well?

B: Yes, it works very well

OR

A: Do police work well?

B: Yes, it work very well

OR

A: Do police work well?

B: Yes, they work very well

Actually it's the same point number 1, but anyway

The police are a group of people, and that overrides the idea of them being an entity, so you have to use a pronoun that can be applied to people - you want they.

You have chosen a difficult example word. "The police" appears singular, but is usually treated as plural, as it refers to a group of people.

Now in your examples you would probably want an article. This is you mean "the institution of the police" not "some individual police officers". We say "The police are here".

The tag question follows the same verb form: "The police are here, aren't they". The word "police" is usually treated as plural, and the plural pronoun is "they".

Moreover we would say "The police work very well", or "They work very well". We are talking about the institution of the police, and the word "police" is usually treated as plural.

If we want to refer to a particular individual we would have to say "That police officer works well", or "he works well".

So "Police" is usually treated as plural, often has the definite article, and can be replaced with the pronoun "they".

To generalize this beyond just the noun "police", when you form a question tag for a statement using some form of "to be", you echo the verb form that was used in the actual question, negated, plus a matching pronoun. Since you said "the police are here", your tag question also uses are (with the appropriate negation). And since are is a plural form, your question tag must also take a plural pronoun.

So any time you say "X are Y", the appropriate question tag is "aren't they?".

For "X is Y", it's one of "isn't he?", "isn't she?", or "isn't it?", depending on the gender of X.

For "X aren't Y", the question tag is "are they?"

For "You are Y", it's "aren't you?" and for "You aren't Y", it's "are you?".

For "I'm not Y", the question tag is "am I?".

The only interesting case is when you are question-tagging a positive first-person statement:

"I am going, ____ __?"

According to the rule I just laid out, you should be saying "amn't I?", but that is no longer accepted English. (You may encounter it in older works but I sincerely doubt you will ever hear anyone use it as part of their standard conversational style.) Instead, the normal tag is "aren't I?". However, you may also hear "am I not?".

The police are here, aren't they?

Police is usually plural, and works like the noun people.

The people are here, aren't they? or: People are here, aren't they?

Generally, in conversation, we say "The police" etc. [regardless of the side of the pond]

However, in the US, you will ***often**** see/hear:

The police is here, aren't they?

I personally would never talk about police without using "the".

For example:

The Virginia State Police is investigating the discovery of two bodies at a crash site along US 23 in Lee County early Sunday morning.

police is

CNN

The police is here to assist and to assure that public order is kept, and that everyone has the right to exercise his or her constitutional rights. That is the reason why law enforcement is here.
COLONEL STANLEY GRIFFIN, LOUISIANA STATE POLICE

More American speech: TV show: The Rookie

- CAPTAIN ANDERSEN: Gentlemen.

- ROB: Oh, you gonna walk because the police is here now?

CAPTAIN ANDERSEN: Gentlemen!! Hey, hey, hey.

The Rookie: the police is here

transcript of tv interview