Final answer:
To transform sentences into the passive voice, the subject becomes the receiver of the action, as in 'Tomatoes are being sold at the market by her.' and 'A test will be written by the teacher.'
Step-by-step explanation:
In English grammar, converting sentences from active voice to passive voice involves making the object of the action into the subject of the sentence. The active sentence "She is selling tomatoes at the market" can be transformed into passive voice as "Tomatoes are being sold at the market by her." For the sentence "The teacher will write a test," the passive version is "A test will be written by the teacher." In the passive voice, the subject receives the action, and the main verb is accompanied by a form of the verb "to be" and the past participle of the main verb.