The thieves have been arrested by the police.

The above sentence is already in passive voice. The active form of this sentence would be:

The police has arrested the thieves. (Active voice)

[Here ‘the police’ is considered as a whole department, therefore ‘has’ is used.]

Otherwise….

The police have arrested the thieves.

Active and Passive Voice

Active voice is when the subject of the sentence performs the action, while passive voice is when the subject receives the action.

Active to Passive Voice

To change an active sentence to passive, follow these steps:

1. Identify the object of the active sentence
2. Move the object to the beginning of the sentence
3. Add the verb "to be" in the appropriate tense
4. Add the past participle of the main verb
5. Add the preposition "by" followed by the subject of the active sentence

For example:

Active: The police have arrested the thieves.
Passive: The thieves have been arrested by the police.

Passive to Active Voice

To change a passive sentence to active, follow these steps:

1. Identify the subject of the passive sentence
2. Move the subject to the beginning of the sentence
3. Identify the verb of the passive sentence and change it to the appropriate tense
4. Add the object of the passive sentence
5. Remove the preposition "by"

For example:

Passive: The thieves have been arrested by the police.
Active: The police have arrested the thieves.

In conclusion, understanding the difference between active and passive voice is important in writing clear and concise sentences. Knowing how to convert between the two can also help to vary sentence structure and improve the flow of writing.