TASK ABOUT ACTIVE AND PASSIVE VOICE

I have task to explain about active and pasive voice, in the grammar text book i find the mean active and passive voice.

•      Active voice  are the sentences with the subject doing something,
•    while the passive voice are sentence with a subject to employment,

in the task i have two for example to see the change sentence from a active voice to passive voice and otherwise.

The example change active sentences to passive sentences : 

-          the secertary is going to answer the letter (active voice) 

-          the letter is being answered by her (passive voice)

the last example change passive sentences to active sentence: 

-          medicine can be prescribed by a doctor (passive voice) 

-          the doctor can prescribe the medicine (active voice)

from the two example we can see the different about active and pasive voice, in the sentence active voice have a subject to doing something, but in passive voice otherwise. We can get the easy conclusion : in the active voice the subject in beginning sentence, because the subject do something, but in passive voice the subject behind the sentence, and the usually clarified with the word” by”