Topic
Sentences in which the error is an incorrect passive are common in written expression questions on the TOEFL test. You therefore need to be able to recognize the correct form of the passive and to be able to determine when a passive verb rather than an active verb is needed in a sentence.
The difference between an active and a passive verb is that the subject in an active sentence does the action of the verb, and the subject in a passive sentence receives the action of the verb. To convert a sentence from active to passive, two changes must be made. (1) The subject of the active sentence becomes the object of the passive sentence, while the object of the active sentence becomes the subject of the passive sentence. (2) The verb in the passive sentence is formed by putting the helping verb be in the same form as the verb in the active sentence and then adding the past participle of this verb.
The first example is an active sentence. To convert this active sentence to a passive sentence, you must first make the subject of the active sentence, Margaret, the object of the passive sentence with by. The object of the active sentence, letter, becomes the subject of the passive sentence. Next, the passive verb can be formed. Because wrote is in the past tense in the active sentence, the past tense of be (was) is used in the passive sentence. Then the verb wrote in the active sentence is changed to the past participle written in the passive sentence.
It should be noted that in a passive sentence, by + object does not need to be included to have a complete sentence. The following are both examples of correct sentences.
The letter was written yesterday by Margaret.
The letter was written yesterday.
Notice that these passive sentences are correct if by Margaret is included (as in the first example) or if by Margaret is omitted (as in the second example).