সহায়িকা

The correct passive voice for "Sheila was writing a letter" is "A letter was being written by Sheila," following the Past Continuous Passive structure (Object + was/were + being + Past Participle + by + Subject) to show an ongoing action in the past.
Here's the breakdown:
Original Sentence (Active): Sheila (Subject) was writing (Past Continuous Verb) a letter (Object).
Passive Structure: Object + was/were + being + Past Participle + by + Subject.
Applying the Rule: A letter (Object) + was + being + written (Past Participle of write) + by + Sheila (Subject).

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