五。指代不清（Ambiguous Reference of Pronouns）
病句1：Mary was friendly to my sister because she wanted her to be her bridesmaid.
改为：Mary was friendly to my sister because she wanted my sister to be her bridesmaid.
病句2：And we can also know the society by serving it yourself.
改为：We can also know society by serving it ourselves.
There are many ways we get to know the outside world.
剖析：这个句子包含了两层完整的意思：“There are many ways.”以及“We get to know the outside world.”。简单地把它们连在一起就不妥当了。
There are many ways for us to learn about the outside world.
There are many ways through which we can become acquainted with the outside world.