**Rule**: For active sentences using 'has to'/'have to' to express necessity, the passive form is **Object + has/have to + be + V3**. The choice between 'has' and 'have' depends on the new subject. - **Correct Answer (c)**: The object 'these papers' becomes the subject. Since 'papers' is plural, it takes 'have to be', followed by the past participle 'signed'. - **Incorrect Answer (a)**: 'have been signed' is the passive of the Present Perfect Tense ('have signed'), which is incorrect. - **Incorrect Answer (b)**: 'are to be' implies a plan or arrangement, which is a different meaning from the necessity expressed by 'has to'. - **Incorrect Answer (d)**: 'had to be' incorrectly changes the tense to the past.

**Rule**: For a Simple Present Tense sentence with two objects (indirect 'us' and direct 'grammar'), either can become the subject of the passive sentence. - **Correct Answer (d)**: Both (a) and (b) are correct passive transformations. - In (a), the direct object 'Grammar' becomes the subject. The passive verb is 'is taught', and the indirect object 'us' is preceded by 'to'. - In (b), the indirect object 'us' becomes the subject 'We'. The passive verb is 'are taught', and the direct object 'grammar' follows. - **Incorrect Answer (a)**: This is correct, but (b) is also correct, making (d) the best choice. - **Incorrect Answer (b)**: This is correct, but (a) is also correct, making (d) the best choice. - **Incorrect Answer (c)**: This incorrectly changes the tense to Simple Past.

**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 cake' is singular, so 'is' is used, followed by the object and the correct passive structure 'going to be baked'. - **Incorrect Answer (a)**: This changes the structure from 'going to' future to 'will' future. - **Incorrect Answer (c)**: This incorrectly changes the tense to the past. - **Incorrect Answer (d)**: 'who' is the incorrect case; 'whom' is required after the preposition 'By'.

**Rule**: When the active sentence uses 'will have to', the passive form is **Object + will have to + be + V3**. - **Correct Answer (b)**: The object 'a new strategy' becomes the subject. The structure 'will have to be found' is the correct passive form for the future necessity. - **Incorrect Answer (a)**: This omits 'have to', changing the meaning to a simple future action. - **Incorrect Answer (c)**: 'would' is an incorrect modal change. - **Incorrect Answer (d)**: 'will have been found' is the passive of the Future Perfect Tense.

**Rule**: Imperative sentences that make a request (often starting with 'Please' or 'Kindly') are converted into passive voice using the phrase **'You are requested to/not to'** followed by the base form of the verb. - **Correct Answer (c)**: The word 'Kindly' indicates a polite request. Therefore, 'You are requested not to...' is the most appropriate passive structure. - **Incorrect Answer (a)**: 'ordered' is incorrect as the original sentence is a request, not a command. - **Incorrect Answer (b)**: The 'Let' structure is less suitable for a polite request. - **Incorrect Answer (d)**: This is an active voice sentence offering advice, not a passive conversion of the original request.

**Rule**: For a negative sentence in the Past Perfect Tense, the passive structure is **Object + had + not + been + V3**. A common alternative is to make the object positive and use 'No' instead of 'not any'. - **Correct Answer (b)**: The phrase 'not any matches' is converted to 'No matches', which becomes the subject. The verb form 'had been won' is the correct passive for the Past Perfect. This structure is more idiomatic than using 'Any matches had not been...'. - **Incorrect Answer (a)**: This structure is grammatically awkward. - **Incorrect Answer (c)**: This incorrectly changes the tense to Simple Past. - **Incorrect Answer (d)**: This incorrectly changes the tense to Simple Past and is also awkward.

**Rule**: When the active sentence uses 'have to' to show necessity, the passive form is **Object + has/have to + be + V3**. The agent 'by them' can be omitted. - **Correct Answer (a)**: The object 'the survey' becomes the subject. Since 'survey' is singular, it takes 'has to be', followed by the past participle 'completed'. The agent 'by them' is correctly omitted. - **Incorrect Answer (b)**: 'had to be' incorrectly changes the tense to the past. - **Incorrect Answer (c)**: This incorrectly changes the tense to the future. - **Incorrect Answer (d)**: 'has been completed' is the passive of the Present Perfect Tense ('has completed'), not 'have to complete'.

**Rule**: For sentences with modal verbs (like must), the passive structure is **Object + modal verb + be + V3**. The agent ('by you') is often omitted when it is obvious. - **Correct Answer (d)**: The object 'Your parents' becomes the subject. The modal 'must' is followed by 'be' and the past participle 'obeyed'. Omitting 'by you' is more natural and concise as it's clearly implied. - **Incorrect Answer (a)**: 'should' is a different modal, implying advice rather than the strong obligation of 'must'. - **Incorrect Answer (b)**: 'are to be' changes the meaning to a plan or arrangement. - **Incorrect Answer (c)**: While grammatically correct, it's less concise than option (d) due to the redundant 'by you'.

**Rule**: For a sentence in the Past Continuous Tense, the passive voice structure is **Object + was/were + being + V3**. The agent 'by them' can be omitted. - **Correct Answer (c)**: The object 'the tickets' becomes the subject. Since 'tickets' is plural, 'were' is used, followed by 'being' and the past participle 'checked'. The agent 'by them' is correctly omitted. - **Incorrect Answer (a)**: This incorrectly changes the tense to Simple Past. - **Incorrect Answer (b)**: This incorrectly changes the tense to Present Continuous. - **Incorrect Answer (d)**: This incorrectly changes the tense to Present Perfect.

**Rule**: For sentences in the Present Continuous Tense with a phrasal verb (verb + preposition), the preposition is retained in the passive voice. The structure is **Object + is/am/are + being + V3 + preposition + by + Subject**. - **Correct Answer (c)**: The object 'the complaint' becomes the subject. The verb form 'is being looked into' correctly reflects the passive voice of the Present Continuous Tense, keeping the preposition 'into' with the verb. - **Incorrect Answer (a)**: This incorrectly changes the tense to Simple Present. - **Incorrect Answer (b)**: This incorrectly changes the tense to Past Continuous. - **Incorrect Answer (d)**: This incorrectly changes the tense to Present Perfect.