When to use Active and Passive sentences
During general communication, one only prefers to convey the meaning and does not usually emphasize one specific subject. It does happen but, general communication covers other factors such as facial expressions, pitch and the use of words, etc. So, it is comparably easy to convey the actual intentions or meanings of a specific speech and the use of passive voice is not very much needed.
In writing, passive voice is generally used to enhance the intensity of a specific subject which the writer wants the reader to focus more on. Consider for example:
AV: Noam Chomsky presented the theory of transformational rules which was a great success.
PV: The theory of transformational rule, which was a great success was presented by Noam Chomsky.
It is apparent that a reader would focus more on the fact that the theory of transformational rules was a great success if it were to be written in Active Voice. On the other hand, if one uses Passive voice, the focus of the reader diverts towards the person who is credited with the success. A writer has the ability to enhance a subject on which he desires the reader to focus more.
Writing is a formal, neutral, and very direct use of words because it is not easy to convey the meaning in writing, as it is in general communication.