The passive of an active tense is formed by putting the verb to be into the same tense as the active verb and adding the past participle of the active verb. The subject of the active verb becomes the ‘agent’ of the passive verb. The agent is very often not mentioned. When it is mentioned it is preceded by BY and placed at the end of the clause.
Active: My grandfather planted this tree.
Passive: This tree was planted by my grandfather.