keisatu ga iriguti wo mihatta.

A natural reading is:

けいさつ が いりぐち を みはった。

• 警察 = けいさつ
• 入口 = いりぐち
• 見張った = みはった

So the whole sentence is:

けいさつ が いりぐち を みはった。

が marks the grammatical subject of the sentence: the one who did the action.

So in:

警察 が 入口 を 見張った。

警察 is the one that did the watching/guarding.

If you changed it to は, the sentence would still be possible, but the nuance would change:

• 警察が入口を見張った。
  = It was the police who watched the entrance. / The police watched the entrance.

• 警察は入口を見張った。
  = As for the police, they watched the entrance.
  This can sound more like a topic, or contrast with something else.

For a simple neutral statement, が is very normal.

Yes, 入口 is a place, but here it is the object of the verb 見張る.

見張る means something like:

• to watch
• to keep watch on
• to guard
• to keep an eye on

So the thing being watched is marked with を.

Compare:

• 入口を見張る = watch the entrance
• 犯人を見張る = watch the criminal
• 建物を見張る = watch the building

Even though 入口 is a location in real life, grammatically it is the thing being watched, so を is correct.

見張った is the past plain form of 見張る.

• dictionary form: 見張る
• past plain form: 見張った

見張る usually means more than just casually looking at something. It often has the sense of:

• guarding
• keeping watch
• standing watch over
• monitoring carefully

So 見張った means:

• watched
• kept watch over
• guarded

depending on context.

Because 見張る is a -u verb ending in る, not an ichidan verb.

This is important: not all verbs ending in る conjugate the same way.

• 見張る is a godan verb
• past tense changes る to った

So:

• 見張る → 見張った
• 座る → 座った
• 取る → 取った

This is different from an ichidan verb like:

• 食べる → 食べた

So even though it ends in る, 見張る follows the godan pattern.

Usually 警察 means the police as an institution or police in general.

In English, the translation may come out as:

• the police
• police

Depending on context, a learner might expect a police officer, but if you want to be specifically clear about an individual officer, 警察官 is more precise.

• 警察 = police / the police
• 警察官 = police officer

That said, in some contexts Japanese can use 警察 in a way that refers to police personnel generally, and English may translate it naturally as a police officer or the police depending on the situation.

Japanese nouns usually do not show singular/plural the way English does.

So 警察 does not itself tell you clearly whether it is:

• one police officer
• multiple police officers
• the police as an organization

You understand that from context.

In many cases, 警察 is best understood as the police rather than a clearly countable singular noun.

Japanese does not use articles like English a, an, and the.

So a sentence like:

警察 が 入口 を 見張った。

can be interpreted in English as:

• The police watched the entrance.
• A police officer watched the entrance.
• The police guarded the entrance.

Which English article sounds best depends on context, but Japanese itself does not mark that distinction directly.

Yes, Japanese word order is more flexible than English, as long as the particles stay correct.

The basic order here is:

警察 が 入口 を 見張った。

But you could also say:

入口を警察が見張った。

This gives more emphasis to 入口.

Because が and を show the grammatical roles, the meaning stays basically the same.
Still, the most neutral and textbook-like order is often:

subject + object + verb

So:

警察 が 入口 を 見張った。

Yes. Here are some related forms of 見張る:

• 見張る = watch / guard
• 見張った = watched / guarded
• 見張らない = do not watch / do not guard
• 見張っている = is watching / is guarding / is on watch

Examples:

• 警察が入口を見張る。
  The police watch the entrance / will watch the entrance.

• 警察が入口を見張らない。
  The police do not watch the entrance.

• 警察が入口を見張っている。
  The police are watching the entrance.

So 見張った specifically tells you the action happened in the past.

In this sentence, yes: 入口 is read いりぐち.

It means:

• entrance
• doorway
• entry

The kanji are:

• 入 = enter
• 口 = opening / mouth / entrance

So 入口 literally suggests an opening where one enters.

There are some words in Japanese whose kanji can have multiple readings in different compounds, but for this common noun, いりぐち is the normal reading.

The structure is:

[subject] が [object] を [verb]

So:

• 警察 = subject
• が = subject marker
• 入口 = object
• を = object marker
• 見張った = verb in past tense

This is a very common Japanese pattern.

You can think of it as:

The police + watched + the entrance

but Japanese places the verb at the end:

警察 が 入口 を 見張った。