The passive voice is a grammatical construction where the subject of the sentence receives the action rather than performing it. The basic structure of a passive sentence is 'object + form of 'to be' + past participle of the main verb'. When a modal verb (like 'can', 'could', 'will', 'would', 'may', 'might', 'must', 'should') is present in the active voice, it is retained in the passive voice, followed by 'be' and the past participle.
The original sentence, "He can solve the problem," is in the active voice, with 'He' as the subject, 'can solve' as the verb phrase, and 'the problem' as the object. To convert this to passive voice, 'the problem' becomes the new subject. The modal verb 'can' is kept, followed by 'be', and then the past participle of 'solve', which is 'solved'. Therefore, the correct passive voice is The problem can be solved.
- The problem could be solved is incorrect because it changes the modal verb from 'can' (present ability) to 'could' (past ability or possibility), altering the original meaning.
- The problem is solved is incorrect because it uses the simple present passive, removing the modal verb 'can' and its meaning of ability.
- The problem has been solved is incorrect because it uses the present perfect passive, which changes the tense and aspect, implying completion rather than ability.