Mary eats a banana
A banana is eaten by Mary

Mary has eaten a banana
A banana has been eaten by Mary

Mary is eating a banana
A banana is being eaten by Mary

"to be" is the auxiliary of the passive voice
be:

We did not play football
Football was not played.

Did you buy the book?
Was the book bought?

Doesn't she like potatoes?
Aren't potatoes liked?

To shift from active voice to passive voice the auxiliary "be" musty agree in tense with the verb of the active voice statement.
Then add the past participle of the active voice statement

she made my book fall.
My book was made to fall.

They say that my cat is dead.
My cat is said to be dead

he will beat John
John will be beaten

He will be beating John
John will be being beaten