The paper is full of words, so we have fewer places to write____ (in/on).

The paper is full of words, so we have fewer places to write_on___ (in/on).,这张纸上写满了单词,所以我们有更少的空间来写上其他的单词,write sth on the paper,把什么写在纸上!

on

write on在...上写
in是指具体的工具，比如write in ink用墨水写

ON
ON:表面接触的
in：在。。。。范围之内

In (因为是in the place)