Exercise is beneficial to your heart. A 22-year study was conducted by doctors in California. They found that people who work at physical jobs experience fewer heart attacks than other people. These active people work all the time at moderate speeds, their daily routine gives them an adequate amount of exercise and helps them stay in shape.
"Exercise is beneficial"这是毫无疑义的。但主题句中如不加上"to your heart"来加以 control 和 limit，那就流于空泛。
因此，紧紧把握主题句中controlling idea的导向和制约作用，是短文写作成功关键之一。
2．推展句
2.1 主要推展句
主要推展句(major supporting statement)的主要特点是：围绕段落主题句展开的每一个推展句本身都不要求作进一步的说明或证明，句与句之间的关系是相互独立又是互相连接的。
例1：(主题句) There are several factors affecting climate. (推展句1)One factor is the mount of sunlight received. (推展句2)Altitude, or the height above sea level, also determines climate. (推展句3)The oceans have an effect on the climate of adjacent land. (推展句4)In addition, climate is influenced by the general circulation of the atmosphere.
主题句指出影响气候的几个因素。然后用四个扩展句说明四种因素。第一种是太阳光的接收量，第二种是海拔高度，第三句和第四句分别是海洋和大气环流因素。
2.2 次要推展句
次要推展句(minor supporting statement)是指对主要推展句作进一步的事实分析和举例说明。它从属于某一个或某几个推展句。
例2：(主题句)I don`t teach because teaching is easy for me. (主要推展句1)Teaching is the most difficult of the various ways I have attempted to earn my living: mechanic, carpenter, writer. (主要推展句2)For me, teaching is a red-eye, sweaty-palm, sinking-stomach profession. (次要推展句1)Red-eye, because I never feel ready to teach no matter how late I stay up preparing. (次要推展句2)Sweaty-palm, because I`m always nervous before I enter the classroom, sure that I will be found out for the fool that I am. (次要推展句3)Sinking-stomach, because I leave the classroom an hour later convinced that I was even more boring than usual.