The ideal vehicle of expression in any language is the sentence. We know that sentences are formed by a verb and some other ingredients such as subject, object, adverb or adjective. A sentence structure depends on its nature. We can say that there are three kinds:
Susan ate an apple. (Subject + Verb + Object)
The apple was eaten by Susan. (Object + BE + Verb in Participle + by Subject)
Eat the apple! (Verb + Object)
The Passive Sentence is formed by an Object + Helping Verb (to be) + Main Verb (past participle) + Complement.
Example: These cars are made in Brazil.
The tense is carried in the helping verb (to be). So if you want to express the same idea in a different tense you would have to conjugate such helping verb.
Here is a video that will explain more details about passive voice with a few examples: