同学你好，该知识点来自沪江网校《新概念英语1、2、3、4册连读标准会话【随到随学班】》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。

同学你好~

She kept it tied to a tree in a field during the day and went to fetch it every evening.
白天，玛丽把羔羊拴在地里的一颗树上，每天晚上把它牵回家。

语法点：
keep+宾语+宾语补足语，宾补一般起补充说明的作用，修饰宾语（不是动词keep）。
keep后的宾补一般由形容词、doing、done、介词短语、副词等担当。如：
We should keep our classroom clean and tidy.
I'm sorry to keep you waiting so long.
He kept his eyes closed to relax himself.
In our daily life,we must keep ourselves away from danger.

这句话里面tied to a tree为过去分词短语，作宾语it的补足语。
过去分词短语作宾语补足语很常用，还有很多动词可以有宾语补足语，比如:
I wanted eggs boiled. 我想把鸡蛋煮着吃。
You'd better had your coat mended. 你最好请人把衣服补一补。
He got his tooth pulled out yesterday. 他昨天把牙拔了。

希望可以帮到你~祝你学习进步~