Here are the sentences transformed into passive voice with explanations for each:

1. **Active:** It is high time we changed all this old furniture.
**Passive:** It is high time that all this old furniture was changed.
**Explanation:** "High time" can be followed by the past subjunctive form, hence we use "was changed."

2. **Active:** They'll arrange for someone to install a new fire alarm in the house.
**Passive:** A new fire alarm will be arranged to be installed in the house.
**Explanation:** "Arrange for someone to" transfers the action to the noun phrase, making it passive.

3. **Active:** It's impossible to force the manager to change his mind.
**Passive:** It’s impossible for the manager’s mind to be changed.
**Explanation:** The emphasis shifts to the manager's mind being changed, thus making it passive.

4. **Active:** I should really mend the fence now.
**Passive:** The fence should really be mended now.
**Explanation:** The subject "I" is omitted for a passive construction, focusing on the action regarding the fence.

5. **Active:** It's urgent that they call a staff meeting.
**Passive:** It's urgent that a staff meeting be called.
**Explanation:** The passive form emphasizes the necessity of the staff meeting being called.

6. **Active:** They have reported that the terrorists have blown up the tunnel.
**Passive:** It has been reported that the tunnel has been blown up by the terrorists.
**Explanation:** The focus is shifted to the tunnel and the action rather than the reporters.

7. **Active:** People say Mr. Johnson is a person of great influence in the company.
**Passive:** Mr. Johnson is said to be a person of great influence in the company.
**Explanation:** This construction expresses the general belief about Mr. Johnson without specifying who says it.

8. **Active:** Someone broke into her flat while she was out.
**Passive:** Her flat was broken into while she was out.
**Explanation:** The focus is now on "her flat" rather than the unknown intruder.

9. **Active:** It's the duty of the police to find the culprits of the murder.
**Passive:** The culprits of the murder are to be found by the police.
**Explanation:** This construction makes it clear that the action of finding is the focus.

10. **Active:** It's compulsory for all workers to wear a helmet on the construction site.
**Passive:** Helmets are compulsory to be worn by all workers on the construction site.
**Explanation:** The emphasis is shifted to the helmets and the requirement for them to be worn.

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