when we write passive voice of a sentence then should we use past  or future tense

Because the subject is being "acted upon" (or is passive), such sentences are said to be in the passive voice. Rules remain unchanged about changing the form of the sentences from active to passive. Few instances would help you understand the concept better:

1. He had left the car at his office changes to The car was left at his office.

2. He will be joining you in a while changes to You shall be joined by him in a while.

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