The passive voice is useful under certain circumstances; for example, if the precise subject of the action is not known ("My car was spray-painted last night") or if the receiver of the action is more important ("The Emperor was assassinated last night"). However, more often than not passive sentences can be improved by reworking the sentence so that the verb is active.
The main problem with passive sentences is that they are wordy: "The theme that was most dealt with by the 16th Century poets was . . ." can easily be condensed into "The 16th Century poets most often dealt with the theme of . . ." This next sentence is virtually incomprehensible because it uses too many passives: "Another illustration of the word 'master' being used to define material things as being the controlling element is . . ."
When you are providing examples, avoid passive constructions such as:
and so on. If you find it necessary to introduce material in this way, use active verbs: "One [we, I] can see that . . ."
Writers often use the passive voice to avoid "I," because they have been told from an early age that "I" is unacceptable. Different academic disciplines, and different instructors within those disciplines, have varying attitudes to the use of the personal pronoun: you would be wise to check. However, the sparing use of "I" can be an alternative to unwieldy passives like some of those cited above: "I think . . ." is both more effective stylistically and more honest than "It is thought that . . ." The passive is often used to avoid responsibility: "The economy of this Province has been mismanaged" is less incriminating than "We have mismanaged the economy of this Province."
Do not shift from active to passive in the same sentence:
As I entered the mansion, footsteps could be heard from behind me.
Both the verb and the subject have shifted. The sentence should read:
As I entered the mansion, I heard footsteps behind me.