Solution:
He was accused of theft by his friends. Given sentence is in simple past tense and it is in active voice. Rule : Subject + (was/were) + V3 + Other agents

Solution:
Let not the police be informed about the robbery.

Solution:
The Nobel Prize for Physics has been won by an American scientist. Given sentence is in present perfect tense and it is in active voice. Rule : Subject + (have/has) + been + V3 + Other agents

Solution:
Let the water be boiled and cooled before it is kept in the fridge.

Solution:
He was refused admittance by the peon. Given sentence is in Past simple tense and it is in active voice, we need to change it into passive voice. Rule : Subject + (was / were) + V3 + Optional Agents.

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Solution:
One's promise should be kept. The subject 'One' has to be replaced with 'One's promises' keeping the tense same.