Change the voice of the following sentence: The teacher advised the students to work hard.

The correct answer is A. The active sentence has the simple past tense (advised), with the teacher as subject, the students as object, and an infinitive complement (to work hard). In the passive, the object becomes the subject, the verb takes the form was/were plus the past participle, and the original subject moves into a 'by' phrase. The infinitive 'to work hard' stays as it is, so the tense stays simple past: 'The students were advised by the teacher to work hard.'

• B is wrong because it converts the infinitive into a that-clause with 'should'. That is a reported restatement, not a voice change of the same sentence, and it alters the structure the question asks you to keep.
• C is wrong on two counts: the continuous passive 'were being advised' changes the tense without any reason, and 'had' should be 'hard'.
• D uses the present perfect passive 'have been advised', which changes the past simple of the original into a present perfect.

The core concept is the rule that voice change keeps the tense and the infinitive complement intact, and only the subject, object and verb form shift. Voice transformation is a standard item in English grammar and language-skill questions, and clear written English matters just as much in Mains answers.

Trap: Students often choose B because it looks grammatically complete, or they pick D because the perfect tense seems more formal. Keep the original tense and the infinitive, and check that only the structure has moved, not the meaning.