keisatu ha musen wo tukau.

• は marks 警察 as the topic: As for the police, ...
• を marks 無線 as the direct object of the verb 使う: it is the thing being used.

So the structure is:

• 警察 は = as for the police
• 無線 を = radio / wireless communication
• 使う = use

は and が are both important particles, but they do different jobs.

In this sentence, は is natural because the sentence is making a general statement about the police:

• 警察は無線を使う。
  Police use radios. / As for the police, they use radios.

If you used が instead:

• 警察が無線を使う。

it would sound more like you are specifically identifying the police as the ones who use radios, perhaps in contrast with someone else.

Very roughly:

• は = talking about a topic, often in a general way
• が = identifying or emphasizing the subject

警察 usually means the police as an institution or group.

If you want to say a police officer, Japanese more often uses:

• 警察官 = police officer

So in this sentence, 警察 is best understood as the police rather than an individual officer.

無線 literally means wireless.

In real usage, it can refer to:

• radio communication
• a radio set
• a police radio
• walkie-talkie-style communication, depending on context

In 警察は無線を使う。, it usually means something like:

• Police use radios
• Police use radio communication

The exact English wording depends on context, but the Japanese is perfectly natural.

使う is the dictionary form of the verb. It is also the plain non-past form.

The Japanese non-past can express:

• a present habitual action
  
  • Police use radios
• a general fact
  
  • Police use radio communication
• sometimes future meaning, depending on context
  
  • The police will use radios

In this sentence, the most natural reading is a general/habitual statement.

By itself, it most naturally means:

• The police use radios
• Police use radios

It sounds like a general fact or habit.

If you specifically want are using, Japanese usually uses 〜ている:

• 警察は無線を使っている。
  The police are using radios.

So 使う and 使っている are not exactly the same.

Japanese usually does not use articles like the or a, and nouns usually do not change form for singular/plural.

So:

• 警察 can mean the police, police, or sometimes a police force, depending on context.
• 無線 can mean a radio, radios, or radio communication, depending on context.

English forces you to choose things like singular/plural and articles much more often than Japanese does.

It is read:

けいさつ は むせん を つかう。

A few pronunciation notes:

• 警察 = けいさつ
• 無線 = むせん
• 使う = つかう
• The particle は is written は but pronounced わ
• The particle を is usually pronounced お

So in speech, it sounds like:

けいさつ わ むせん お つかう。

Yes, it is a normal and understandable expression.

使う is a very common verb meaning to use, so 無線を使う simply means to use radio/wireless communication.

Depending on context, Japanese might also use more specific wording, but this sentence is grammatically natural and easy to understand.

Yes. Japanese often omits subjects and topics when they are obvious from context.

For example, in the right context, someone might just say:

• 無線を使う。
  (They/I/we) use radios.

But when you want to clearly say who you are talking about, you include 警察は.

So here, 警察 is included to make the topic explicit.

The basic order is:

topic + object + verb

So:

• 警察は = topic
• 無線を = object
• 使う = verb

Japanese verbs usually come at the end of the sentence, unlike English.

That is why 使う comes last.