spend as much time, if not more, studying by themselves as being taught 应该加逗号隔开,就清楚了。本句涉及两个问题: 1. as...as... 一样多。 2. spend time doing 花费时间做某事。
把这两种结构合理地柔和在一起,就是:spend as much time doing A as doing B.
句子类比: Jim plays football as well as, if not better than, Mike. 吉姆踢足球如果不是比迈克踢得更好,至少也是一样好。 She sings as well as, if not better than, her sister. 她唱歌如果不比她姐姐好,至少也跟她姐姐一样。 如果不省略,则非常啰嗦,重复很多成分。即: She sings as well as her sister if she does not sing better than her sister.