fix look 比较He lay there with his eyes looking at the sky.The

look 作状语,和逻辑主语之间是主动关系,所以用looking; fix one'seyes on sh.短语中我们可以看到one's eyes 和fix之间是动宾关系,所以当fix on做their eyes的定语的时候,fix要用过去分词,来表示他们之间的动宾（被动）关系,这句话还可以这么说：
The students are listening to the teacher ,fixing their eyes on the blackboard.
瞧,这里fix和逻辑主语之间是主动关系,就可以用现在分词形式了.

fix one＇s eye on
改为被动为 one's eyes are fixed on sth
look at sth