No rule says you can’t use “to be” verbs like “are” and “were” or start a sentence with “It is” or “There are.” Likewise, no rule says you can’t use gerunds, verbs ending with –ing like “running.” But we sometimes fall into these constructions without looking for other options. See how many “to be” and “-ing” verbs you can eliminate—with what gain in voice.
First version: There was a time when I believed that if it was in a book, it must be true.
Edited: I once believed all books tell the truth.
First version: We were dancing and talking all night.
Edited: We danced and talked all night. (Context and rhythm matter, so use your ear.)