1. When the agent of the action is unknown:
My wallet was stolen last night. (we don’t know who stole the wallet)
2. When the agent is unimportant:
The new students’ centre was completed last week. (the people who built the centre are unnecessary information for the meaning of the sentence)
3. When the agent of the action is obvious from the context:
I was born in March of '55. (Everyone knows that it was my mother
bore me then)
4. To emphasize (put importance on) the recipient (receiver) of the action:
a. Only Jane was injured in the accident; the remainder of the passengers were unhurt.(we want Jane to be the subject of the sentence and at the beginning to emphasize her importance)
b. Erina was chosen as best student, and of course this made her happy. (the teacher who chose Erina is not what we want to emphasize)
5. To connect ideas in different clauses more clearly:
a. Pharmacologists would like to study the natural ‘pharmacy’ known as the rainforest, if this can be done before clear-cutting destroys it. (in this sentence, keeping THIS near the first clause makes the sentence’s meaning clearer)
b. The music was being played too loud by the students, who were finally asked to turn it down.
6. To make generic statements, announcements, and explanations:
a. Something should be done about the traffic jams in this town.
b. Patrons are asked not to smoke.
c. It's said that it's going to rain tonight.(Often, people will say, 'They say that it's going to rain tonight', the they being the weatherman.)