I've been cautioned to avoid passive sentences with "was." But is every use of "was" passive?

Passive verbs always include some form of the verb “to be” (including “was”), but that doesn’t mean all uses of “was” are passive. That’s a bit like saying all cakes include eggs; therefore, any recipe with eggs will result in a cake. Like the egg, “was” has a variety of uses. For example, it can act as a linking verb:

He was a teacher.

And it can be part of a past progressive construction, which shows something happening in the past at the same time something else happened:

She was running down the stairs when the subway pulled into the station.

So how do you know when a sentence is passive? When the subject doesn’t perform the action; instead the action is done to the subject. For example:

He was chosen by the team.

That same sentence would become active by making the subject the do-er of the action:

The team chose him.

While “was” is commonly seen in passive voice, other forms of the verb “to be” can create passive voice, too. For example:

The peanuts will be shelled by Ann.

(Active: Ann will shell the peanuts.)

So don’t automatically implicate “was” when it comes to determining passive voice. Look at the role of the subject: Is it doing the action? Or is the action being done to it?