A passive voice sentence must have an object. Here's an active sentence with no object:
We ran.
There's no way to make that passive without adding something.
Let's think back to the earlier sentence:
A letter was mailed by me.
It has the verb was in it, and many people will look at that sentence and say, "Ah-ha! It's in passive because it has was." But the was isn't the reason; it's just a clue. The reason is that the object of the action—the letter—is in the subject position. The word was is just a clue.
Here's an active sentence that uses was:
I was running.
I, the person doing the running, is still in the subject position, just like the active sentence before: We ran.
We ran is in the simple past tense, and I was running is in what's called the past continuous tense.
Whereas the simple past tense is used to talk about something that happened in the past, the past continuous tense is used to talk about something that happened in the past but was interrupted. It's more obvious in longer sentences. Here are some examples:
We were running when I tripped.
We were running when it started to rain.
We were running when Bob called on my cell phone.
The was in those sentences has nothing to do with active or passive voice. It's what's called a helping verb or an auxiliary verb, and it comes before the main verb in its participle form: running.
How to Recognize a Passive Voice Sentence
The point today isn't to learn about all the verb tenses though, it's just to make sure you know that when you're trying to figure out if a sentence is in the passive voice, the questions to ask are "Is the object in the subject position?" and “Can I add ‘by so-and-so’ to the end?” not "Does the sentence have was or were or is in it?"
All passive sentences do have a form of the verb to be such as was or were or is in them, but not all sentences that have those verbs are passive.
I got my "always have a form of the verb to be in them" information from a couple of credible university writing center sites.
