it is not known for certain if the activity of a man's mind occurs when he is asleep
问题描述：
it is not known for certain if the activity of a man's mind occurs when he is asleep
这句话怎么理解?有什么句子成分构成?
答
it is not known for certain //if the activity of a man's mind occurs when he is asleep后面是宾语从句
when he is asleep是时间状语从句
全部的意思是：这是不确定的,人的意识是否会产生,在他睡眠的时候.