They did not build Rome in a day.
Here's how we arrived at that:
* Identify the passive voice: The original sentence uses the passive voice because the object ("Rome") is the subject of the sentence.
* Find the implied actor: The phrase "in a day" implies that someone built Rome, but the sentence doesn't specify who.
* Rewrite with an active subject: We use a general, nonspecific pronoun ("they") to represent the builders of Rome.
* Construct an active sentence: "They" becomes the subject, "built" is the verb, and the rest of the sentence follows.