The simple past is a verb tense. It expresses an action that happened in the past, like this,
 Yesterday, I ate cake.
The passive is when the object of the verb is moved to the front of the sentence, like this,
 Yesterday, cake was eaten by me.
A passive verb is formed by using the verb BE (e.g., is, am, was, were, etc) and the past participle of the verb. In our example  the passive verb is "was eaten".
Here's the basic information for the verb To eat:
Present Tense: eat
Past Tense: ate
Past participle: eaten
Present participle: eating
When you learn a verb, any verb, you should learn its four forms: its present tense form, its past tense form, its past participle, and its present participle.
Hope that helps. ;)
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