• Active and Passive sentence in Past form

Active and Passive Voice – Simple Past Tense 
Active sentences in the simple past tense :
Subject + past tense form of the verb + object

Passive sentences in the simple past tense have the following structure: 

Object of the active sentence + was/were + past participle form of the verb + by + subject of the active sentence

Changing an assertive sentence into the passive

Active: He wrote a letter.
Passive: A letter was written by him.

Past Continuous Tense

Active sentences in the past continuous tense have the following structure:
Subject + was/were + -ing form of the verb + object
 
Passive sentences in the past continuous tense have the following structure:Object of the active sentence + was/were + being + past participle form of the verb + by + subject of the active sentence

Examples are given below:

Active: She was writing a novel.
Passive: A novel was being written by her.
Active: They were saying their prayers.
Passive: Their prayers were being said by them.

Past perfect tense

Active sentences in the past perfect tense have the following structure:

S + had + past participle + direct object

passive sentences in the past perfect tense have the following structure:

S (direct object) + had + been + past participle +/- by … (agent)

Example :

Active : They had used poison gas in World War I.

Passive : Poison gas had been used in World War I.