I helped them change into passive voice

They were helped (to) change by me.

Explanation:

In grammar, the term voice refers to the relationship between the action expressed by the verb and the participants (subject and object) of the event described in the sentence. In English, there are two types of voice:

When converting active voice into passive voice, we need to turn the object of the active sentence into the subject of the passive sentence. The tense used in both sentences should be the same. The subject of the active sentence becomes the object of the preposition by, which is used to express the agent (the person or thing who performs an action). This prepositional phrase (by + agent) is located at the end of the sentence.

This is how we'll get the sentence They were helped (to) change by me. The infinitive marker to can be omitted, as it is in the original sentence, but it can be inserted as well. To help someone do something and to help someone to do something means the same thing. Using passive voice in sentences like this isn't ideal, and the sentence sounds a bit less awkward if we insert to.

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