A letter is not written by him into active

He did not write a letter.

Explanation:
To change a sentence from passive voice to active voice, rearrange the sentence so that the subject who is performing the action is at the beginning of the sentence. In the sentence provided, “a letter is not written by him”, the subject who is performing the action is the “him”.

A letter is not written by him.
He did not write a letter.

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