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active:
I enjoyed taking the children to the zoo.

in this clasue I enjoyed .

passive
- The children enjoyed being taken to the zoo.

in this cluse who does enjoyed?
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In your second sentence the children enjoyed themselves.
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Nader75active:
I enjoyed taking the children to the zoo.

in this clasue I enjoyed .

passive
- The children enjoyed being taken to the zoo.

in this cluse who does enjoyed?

Hi Nader75

You have two different active sentences. The verb enjoy is in the active voice in both sentences.
In the first sentence, I enjoyed an activity.
In the second sentence, the children enjoyed an activity.

The difference is in the formation of the gerund after the verb enjoy.
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Comments  
Notice also that in your second sentence "I" doesn't appear. Therefore, "the children" are the only ones who COULD enjoy it!
 Yankee's reply was promoted to an answer.
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Here I enjoyed not the children right..

So how we said children enjoyed