Passive verbs are used in the passive voice, where the emphasis is on what is being done to the object of the sentence. The key word by can often be used to identify the passive voice. Any tense can be used with passive verbs.
The students were being taught by the teacher.
Notice the word by, which clues us in to the passive voice.
The dog was being walked by the woman.
The essay should have been written by you, not by your friend.
The fire department has been notified.