jǐngchá zài zhèlǐ.

Because this is a location sentence, not an A = B sentence.

In Mandarin:

• A 是 B is used when one thing equals or is identified as another thing.
  
  • 他是警察。 = He is a police officer.
• A 在 B is used when someone or something is located at a place.
  
  • 警察在这里。 = The police officer / police are here.

So 警察在这里 is the natural way to say that the police are at this place.

Here, 在 means to be at / to be in / to be located at.

It can act like a preposition in some sentences, but in this sentence it is the main verb of location.

So the pattern is:

• Subject + 在 + Place

Examples:

• 我在家。 = I am at home.
• 老师在学校。 = The teacher is at school.
• 警察在这里。 = The police are here.

The basic order is:

• Subject + 在 + Place

So:

• 警察 = the police / police officer
• 在 = is/are at
• 这里 = here

This is very common in Mandarin for talking about location.

English often says X is here, but Mandarin usually expresses that as X is at here/this place, using 在.

It can mean either, depending on context.

警察 does not change form for singular or plural, so it could mean:

• a police officer
• police officers
• the police

Context tells you which one is meant.

If you want to be clearly singular, you can say:

• 一个警察在这里。 = A police officer is here.

If the situation already makes it obvious, just 警察在这里 is fine.

Both are correct.

• 这里 is a little more neutral and standard.
• 这儿 is more colloquial, especially in northern speech.

So these both work:

• 警察在这里。
• 警察在这儿。

They mean essentially the same thing: The police are here.

Yes. That is a very helpful way to think about it.

• 这 = this
• 里 = inside / place / within

So 这里 literally points to this place, and in natural English that becomes here.

Related words:

• 那里 = there
• 哪里 = where

Examples:

• 他在那里。 = He is there.
• 你在哪里？ = Where are you?

Not exactly. Both are grammatical, but the focus is different.

• 警察在这里。 = The police are here.
  
  • Focus: where the police are
• 这里有警察。 = There is a police officer / there are police here.
  
  • Focus: the existence of police here

So:

• Use 警察在这里 when the police are already the topic.
• Use 这里有警察 when you are introducing the fact that police are present.

No. That would not be a normal grammatical sentence.

For location, Mandarin usually needs 在:

• 警察在这里。 = correct

Without 在, the relationship between 警察 and 这里 is unclear.

So for this kind of sentence, keep the pattern:

• Subject + 在 + Place

In careful dictionary-style pronunciation, 里 is third tone: lǐ.

But in natural speech, 这里 is often pronounced more lightly, almost like:

• zhèlǐ
• or even with a weakened second syllable in fast speech

For a learner, saying zhèlǐ clearly is perfectly fine.

The full pinyin in your sentence is:

• 警察 = jǐngchá
• 在 = zài
• 这里 = zhèlǐ

Yes. Very naturally.

Depending on context and tone of voice, 警察在这里。 can simply state location, or it can sound urgent:

• The police are here.
• The police are here!

In actual speech, people might also say:

• 警察来了！ = The police have come!
• 警察在这儿！ = The police are here!

So your sentence is completely natural in that kind of situation too.