**Rule**: When the active sentence uses 'had to' to show past necessity, the passive form is **Object + had to + be + V3**. - **Correct Answer (d)**: The object 'a new chairman' becomes the subject. The structure 'had to be selected' is the correct passive form for the past necessity. - **Incorrect Answer (a)**: 'had been selected' is the passive of the Past Perfect Tense ('had selected'), not 'had to select'. - **Incorrect Answer (b)**: 'was to be' implies a past plan, which is a different meaning. - **Incorrect Answer (c)**: 'has to be' incorrectly changes the tense to the present.

**Rule**: For a Present Perfect Tense sentence, the passive structure is **Object + has/have + been + V3 + by + Subject**. - **Correct Answer (a)**: The direct object 'a letter' becomes the subject. Since it's singular, 'has' is used, followed by 'been' and the past participle 'written'. The prepositional phrase 'to her friend' follows. - **Incorrect Answer (b)**: This incorrectly changes the tense to Simple Past. - **Incorrect Answer (c)**: This incorrectly changes the tense to Simple Present. - **Incorrect Answer (d)**: This structure, making the indirect object the subject, is less common when the indirect object is part of a prepositional phrase ('to her friend').

**Rule**: When the active sentence uses 'had to' to show past necessity, the passive form is **Object + had to + be + V3**. - **Correct Answer (d)**: The object 'the proposal' becomes the subject. The structure 'had to be rejected' is the correct passive form for the past necessity. - **Incorrect Answer (a)**: 'had been rejected' is the passive of the Past Perfect Tense ('had rejected'), not 'had to reject'. - **Incorrect Answer (b)**: 'was to be' implies a past plan, which is a different meaning. - **Incorrect Answer (c)**: 'must be' incorrectly changes the tense and meaning to present obligation.

**Rule**: When the active sentence uses 'has to'/'have to', the passive form is **Object + has/have to + be + V3**. The choice between 'has' and 'have' depends on the new subject. - **Correct Answer (d)**: The object 'new staff' becomes the subject. 'Staff' is a collective noun that can be treated as plural, so it takes 'have to be', followed by the past participle 'recruited'. - **Incorrect Answer (a)**: 'has' is incorrect because 'staff' is treated as plural here. - **Incorrect Answer (b)**: 'have been recruited' is the passive of the Present Perfect Tense. - **Incorrect Answer (c)**: 'are to be' implies a plan, which is a different meaning.

**Rule**: For an interrogative sentence in the Future Perfect Tense starting with 'Who', the passive structure is **By whom + will/shall + have been + V3?** - **Correct Answer (b)**: 'Who' changes to 'By whom'. The Future Perfect passive verb 'will have been finished' is used correctly with the object 'the work'. - **Incorrect Answer (a)**: This incorrectly uses the Future Simple passive form ('will be finished'). - **Incorrect Answer (c)**: 'would' is an incorrect modal change. - **Incorrect Answer (d)**: This word order is not standard for formal interrogative sentences.

**Rule**: When the active sentence uses 'has to' to show necessity, the passive form is **Object + has/have to + be + V3**. - **Correct Answer (d)**: The object 'a new manager' becomes the subject. Since 'manager' is singular, it takes 'has to be', followed by the past participle 'hired'. - **Incorrect Answer (a)**: 'has been hired' is the passive of the Present Perfect Tense. - **Incorrect Answer (b)**: 'is to be' implies a plan, not the necessity of 'has to'. - **Incorrect Answer (c)**: 'had to be' incorrectly changes the tense to the past.

**Rule**: For active sentences using the 'be going to' future form, the passive structure is **Object + is/am/are + going to be + V3**. - **Correct Answer (c)**: The object 'a new library' becomes the subject. The structure 'is going to be built' is the correct passive conversion. - **Incorrect Answer (a)**: This incorrectly changes the tense to Simple Present. - **Incorrect Answer (b)**: This changes the structure to the 'will' future, which is not a direct conversion. - **Incorrect Answer (d)**: This is the passive of the Present Continuous tense ('is building'), not the 'going to' future.

**Rule**: For a sentence in the Present Perfect Tense, the passive voice structure is **Object + has/have + been + V3 + by + Subject**. - **Correct Answer (d)**: The object 'a delicious meal' becomes the subject. Since it is singular, 'has' is used, followed by 'been' and the past participle 'cooked'. - **Incorrect Answer (a)**: This incorrectly changes the tense to Simple Past. - **Incorrect Answer (b)**: This incorrectly changes the tense to Simple Present. - **Incorrect Answer (c)**: This incorrectly changes the tense to Past Perfect.

**Rule**: For an interrogative sentence with the 'be going to' future form, the passive structure is **Wh- word + is/am/are + object + going to be + V3?** - **Correct Answer (a)**: 'When' remains at the beginning. The object 'the work' is singular, so 'is' is used, followed by the object and the correct passive structure 'going to be finished'. - **Incorrect Answer (b)**: This changes the structure to the 'will' future. - **Incorrect Answer (c)**: This incorrectly changes the tense to the past. - **Incorrect Answer (d)**: The verb 'are' is incorrect because the subject 'work' is singular (uncountable).

**Rule**: When the active sentence uses 'had to' with a phrasal verb, the passive form is **Object + had to + be + V3 + particle**. - **Correct Answer (c)**: The object 'several employees' becomes the subject. The structure 'had to be laid off' is the correct passive form for the past necessity, keeping the particle 'off' with the verb. - **Incorrect Answer (a)**: 'had been laid off' is the passive of the Past Perfect Tense. - **Incorrect Answer (b)**: 'have to be' incorrectly changes the tense to the present. - **Incorrect Answer (d)**: 'were to be' implies a past plan, which is a different meaning.