动词-ing形式作主语表示抽象化的概念，可以是单独的动词，如talking/swimming/sleeping，也可以是动词短语，如learning English / playing the piano / shopping online。
注意，无论是单个动词还是动词短语作主语，后面的谓语一律用单数。
Example: Swimming is a great way to stay in shape. 游泳是保持身材的好方法。
Example: Playing the piano is the passion of my life. 弹钢琴是我生活中最热爱的事。
Example: Shopping online is very convenient because there are many choices at a good price. 网购真的很方便，因为选择多，价格也合适。
