同学你好，该知识点来自沪江网校《2019年6月CET英语四级【全程班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学好，
it being hot,是现在分词做状语,有它自己的主语即it,we went for a swim in the river是句子的主句.

像这种现在分词做状语的情况,如果主句的主语跟从句的主语相同时,从句的主语可以省略；如果不同,则要加上从句的主语.
例句：It being hot,we went for a swim in the river.
这里主从句主语不一致，从句主语不省略。

又如：Living with his parents,he is very happy.
Living with his parents的主语跟主句主语是一样的,所以可以省略.

祝进步。