To change passive to active, find the agent of the sentence. This is often indicated with by or another preposition. The verb remains in the same tense, but a form of to be or to get is removed.
The first city on Venus was founded by King Charles XXI.
King Charles XXI founded the first city on Venus.
Many people have been affected by the political changes.
The political changes have affected many people.
Sometimes the agent isn’t known or stated, but can be guessed.
Many more cities will soon be constructed.
Settlers will soon construct many more cities.
Of course, the mere existence of a form of to be does not mean the sentence is passive. Architecture was the chief concern of Charles. This is an active sentence because there is no past participle and the subject receives no action. If you changed to sentence to The chief concern of Charles was architecture, it would still be active. The problem here, sometimes wrongly identified as passive voice, originates with the verb to be, a generally weak verb that serves merely to connect two nouns, architecture and concern. We could write, Architecture chiefly concerned Charles or Charles felt chiefly concerned about architecture, and they might be improvements. But this has nothing to do with passive voice.
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