Cricket was being played by boys.

Given sentence is in past continuous tense and it is in active voice.

Rule :
Subject + (was/were) + being + V³ + Or agents

Cats are always chased by dogs.

Given sentence is in simple present tense and it is in active voice.

Rule :
Subject + (is/am/are) + V³ + Or agents

The prices were lowered by shopkeeper.

Given sentence is in Past simple tense and it is in active voice, we need to change it into passive voice.

Rule :
Subject + (was / were) + V3 + Optional Agents.

He was given a reward by king.

Given sentence is in Past simple tense and it is in active voice, we need to change it into passive voice.

Rule :
Subject + (was / were) + V³ + Optional Agents.

Let this be done by me.

If a sentence is started with Let passive voice of that sentence also be started with Let.

Rule :
Let +Subject +be + V³ + Or agents.

English is spoken all over world.

Given sentence is in Present simple tense.

Rule :
Subject + (is /am /are) + V³+ Or agents.

The union leader's proposals could not be accepted by manager.

The given sentence contains one of Model verb (Model Verb = will, shall, can, may, might, could, might, must, would). It is in active voice and a negative sentence.

Rule :
Subject + Model verb +not + be + V3 + Optional Objects.

Why did y refuse him admittance?

Given sentence is in simple past tense and it is in passive voice.

Rule :
Why + Subject + V¹ + Or agents + ?

We had completed our task before sunset.

The given sentence is in passive voice and it is in Past Perfect Tense.
To convert it into active voice, we just remove been from given sentence and object (our)will be change into subject (We).

Rule :
Subject + had + V³ + Or agents.

It is time to eat our food.

Given sentence is in Simple future tense and it is in passive voice.

Rule :
Subject + V¹ + Or agents