**Rule**: When the active sentence uses 'had to' to show past necessity, the passive form is **Object + had to + be + V3**. The agent 'by them' can be omitted. - **Correct Answer (c)**: The object 'the meeting' becomes the subject. The structure 'had to be cancelled' is the correct passive form for the past necessity. The agent 'by them' is correctly and naturally omitted. - **Incorrect Answer (a)**: 'had been cancelled' is the passive of the Past Perfect Tense ('had cancelled'), not 'had to cancel'. - **Incorrect Answer (b)**: 'was to be' implies a past plan, which is a different meaning from past necessity. - **Incorrect Answer (d)**: 'has to be' incorrectly changes the tense to the present.

**Rule**: For a Past Perfect Tense sentence, the passive voice structure is **Object + had + been + V3 + by + Subject**. Adverbs like 'already' are typically placed between 'had' and 'been'. - **Correct Answer (d)**: The object 'the topic' becomes the subject. The verb form 'had already been explained' is the correct passive construction. - **Incorrect Answer (a)**: This incorrectly changes the tense to Simple Past. - **Incorrect Answer (b)**: This incorrectly changes the tense to Present Perfect. - **Incorrect Answer (c)**: This incorrectly changes the tense to Simple Present.

**Rule**: For an interrogative sentence with a past modal starting with 'Who', the passive structure is **By whom + modal + have been + V3?** - **Correct Answer (c)**: 'Who' becomes 'By whom'. The past modal passive form 'could have been written' is used. The word order is correct for a question, with the modal 'could' coming before the subject. - **Incorrect Answer (a)**: 'could be written' is the passive for a present modal. - **Incorrect Answer (b)**: The word order is incorrect; the subject 'such a beautiful poem' should come between 'could' and 'have'. - **Incorrect Answer (d)**: 'can' is a different modal.

**Rule**: For an interrogative sentence in the Simple Past Tense starting with a 'Wh-' word, the passive structure is **Wh- word + was/were + object + V3 + by + subject?** - **Correct Answer (d)**: The 'Wh-' word 'How' remains at the beginning. The object 'the solution' is singular, so 'was' is used, followed by the past participle 'found'. - **Incorrect Answer (a)**: This incorrectly changes the tense to Present Perfect. - **Incorrect Answer (b)**: This incorrectly changes the tense to Simple Present. - **Incorrect Answer (c)**: This incorrectly changes the tense to Past Continuous.

**Rule**: For an interrogative sentence with the 'be going to' future form starting with 'Who', the passive structure is **By whom + is/am/are + object + going to be + V3?** - **Correct Answer (b)**: 'Who' changes to 'By whom'. The object 'the plants' is plural, so 'are' is used, followed by the object and the correct passive structure 'going to be watered'. - **Incorrect Answer (a)**: This changes the structure from 'going to' future to 'will' future. - **Incorrect Answer (c)**: The verb 'is' is incorrect because the subject 'plants' is plural. - **Incorrect Answer (d)**: 'who' is the incorrect case; 'whom' is required after the preposition 'By'.

**Rule**: For a negative sentence in the Past Continuous Tense, the passive voice structure is **Object + was/were + not + being + V3 + by + Subject**. - **Correct Answer (c)**: The object 'such a result' becomes the subject. The passive verb form 'was not being expected' correctly reflects the negative Past Continuous tense. - **Incorrect Answer (a)**: This incorrectly changes the tense to Simple Past. - **Incorrect Answer (b)**: This incorrectly changes the tense to Present Perfect. - **Incorrect Answer (d)**: This incorrectly changes the tense to Present Continuous.

**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 'all the details' becomes the subject. Since 'details' is plural, it takes 'have to be', followed by the past participle 'checked'. - **Incorrect Answer (a)**: 'have been checked' is the passive of the Present Perfect Tense. - **Incorrect Answer (b)**: 'are to be' implies a plan, not the necessity of 'have to'. - **Incorrect Answer (c)**: 'had to be' incorrectly changes the tense to the past.

**Rule**: For an interrogative sentence in the Simple Past Tense starting with 'Which', the passive structure is **Which + noun + was/were + V3 + by + subject?** - **Correct Answer (c)**: 'Which route' remains at the beginning. As 'route' is singular, it is followed by 'was' and the past participle 'taken'. - **Incorrect Answer (a)**: This incorrectly changes the tense to Simple Present. - **Incorrect Answer (b)**: This incorrectly changes the tense to Present Perfect. - **Incorrect Answer (d)**: This incorrectly changes the tense to Past Continuous.

**Rule**: For a sentence in the Simple Past Tense, the passive voice structure is **Object + was/were + V3 + by + Subject**. - **Correct Answer (b)**: The object 'several trees' becomes the subject. Since 'trees' is plural, 'were' is used, followed by the past participle 'uprooted'. - **Incorrect Answer (a)**: This incorrectly changes the tense to Simple Present. - **Incorrect Answer (c)**: This incorrectly changes the tense to Past Continuous. - **Incorrect Answer (d)**: This incorrectly changes the tense to Present Perfect.

**Rule**: For an interrogative sentence in the Simple Past Tense starting with 'Did', the passive structure is **Was/Were + object + V3 + by + subject?** - **Correct Answer (c)**: The object 'your proposal' is singular, so the sentence starts with 'Was'. This is followed by the object and the past participle 'approved'. - **Incorrect Answer (a)**: This incorrectly changes the tense to Present Perfect. - **Incorrect Answer (b)**: This incorrectly changes the tense to Simple Present. - **Incorrect Answer (d)**: This is not in the correct interrogative word order; the auxiliary verb 'Was' must come before the subject.