通过范文学习写作－原因模式
范文：
In recent decades, cities have grown so large that now about 50% of the Earth's population lives in urban areas. There are several reasons for this occurrence. First, the increasing industrialization of the nineteenth century resulted in the creation of many factory jobs, which tended to be located in cities. These jobs, with their promise of a better material life, attracted many people from rural areas. Second, there were many schools established to educate the children of the new factory laborers. The promise of a better education persuaded many families to leave farming communities and move to the cities. Finally, as the cities grew, people established places of leisure, entertainment, and culture, such as sports stadiums, theaters, and museums. For many people, these facilities made city life appear more interesting than life on the farm, and therefore drew them away from rural communities.

分析：
Notice how each supporting sentence is a cause that explains the effect mentioned in the topic sentence. In the chart below are the main ideas of the above paragraph, to help you understand the relationships better:

EFFECT
(Topic Sentence) CAUSES
(Supporting Sentences)
Cities have grown very large.
[There are several reasons for this.] Factory jobs attracted people.
(Cities have grown very large.) Better schools attracted families to move
to the city.
(Cities have grown very large.) Places of leisure, entertainment, and culture
made city life appear more interesting.

Notice also how the topic sentence is followed by the "focusing" or "prediction" sentence, There are several reasons for this. Such sentences help the reader anticipate the organization of the paragraph or essay.

Cause and Effect Conjunctions

Here are some common conjunctions that can be used to express cause and effect:

since as a result because of + noun phrase
because therefore due to + noun phrase
consequently for this reason so

There are two things you must be careful of when using these conjunctions. First, you must order the cause and the effect corerctly. For example, in the sentence