Rule 1. Identify the (S+V+O) Subject, Verb and object in the active sentence to convert to passive voice
Example: He drives car. (Subject – He, verb – Drives, object – Car)

Rule 2. Interchange the object and subject with each other, i.e. object of the active sentence become the subject of the passive sentence.
Example : Active voice : She knits sweater. (Subject – She, Verb – Knits, Object – Sweater)

Passive Voice : The sweater is knitted by her. (Object sweater is interchanged with the subject She).
Rule 3. In passive voice sometimes the subject is not used, i.e. the subject in passive voice can be omitted if the sentence without it gives enough meaning.
Example : Milk is sold in litres