How to spot it
Look for a form of be (is, was, were, has been, will be) plus a past participle (broken, eaten, written). Then ask: is the subject doing the action? If not, it is passive. A handy test: if you can add "by zombies" after the verb and it still makes grammatical sense, it is passive.
Why editors flag it
Passive sentences are often longer and can hide who did what (Mistakes were made). Active voice is usually clearer and more direct.
When passive is the right choice
When the doer is unknown or unimportant (My bike was stolen), when the receiver is the topic (Penicillin was discovered in 1928), and in scientific methods sections where the procedure matters more than the person.
Not everything with "was" is passive
She was tired and He was running are active: nobody is receiving an action.
Frequently asked
Is passive voice grammatically wrong?
No. It is a style choice. It becomes a problem when it makes writing vague or wordy.
Is "was" always passive?
No. Passive needs be + past participle, and a subject that receives the action.