一. 双名词从句式

Another reason why people from the countryside transfer to the city is because of the modern conveniences that city living brings.

解析：适用于引入原因(带标题要害词)

句型结构：主语从句+ is because of the 原因 +宾语从句 (包含主从，宾从两个名词性从句)

例句中，people from the countryside transfer to the city 是对标题要害信息的复述，句型中“原因”部分运用一个名词或名词词组。

Exercise: causes of congestion in megacities。

二.三组P+O式

Today more and more people rely on cars instead of walking, have less physical demands at work and prefer inactive leisure activities. This results in burning less calories and gaining weight.

解析：适用于解说句，阐明三种详细原因或定论段总结原因和成果

句型结构：主语+ P.O.1 , P.O.2 and P.O.3 . This results in doing sth.1 and doing sth.2.

例句中，三组P+O (谓语+宾语)是平行结构，第二句的”This” 指代三组P+O; doing sth.1 是result，doing sth.2是ultimate result。

Exercise: causes of overpopulation。

三.“原因+细化方面解说” 式

Effluents are another by-product of industries which poses threat to the environment; leather and tanning industries, petroleum industries and chemical manufacturing industries create major waste products which are released directly into nearby streams without treatment, creating river pollution and causing harm to aquatic life.

解析：适用于阐明一个原因(观点)后紧接着把这个原因的几个方面详细化论说。

句型结构：原因 is another by-product of … which poses threat to 主题词; 详细方面1，详细方面2 and 详细方面3 create … which are V+ed ….without…, V+ing …..to …

例句中，which are…的部分是定语从句，Ving…to…对错谓语用法。