1. A letter was being written by him. 2. The room had been cleaned by us. 3. He is known to me. 4. The homework has been done by me. 5. The exercise had been done by me.
Hope this helps! Remember that you are using passive voice when the subject is being acted upon by the verb.
I don't have time to do the whole essay here's an idea I guess:
I think that the United States should adopt a digital currency due to recent events of the pandemic and it would be more efficient. Cons could be that the system they use could get infiltrated but I doubt that could happen.
