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同学好：
作文修改如下，请参考——
... spent the whole morning picking blackberries ...
spend time (in) doing sth. 表示“在某事上面花时间”，是固定结构。
... sky, and there was no one except herself.
两个分别独立完整的句子在没有连词的时候，不可以写进一个句号；这里指“从整体中排除同类”用except。
... to have a rest and have her lunch before going back to work.
have her lunch 是和have a rest 并列的，用and 连接；before 是介词，后跟动名词做宾语。
... in which there was a sandwich ...
there be 的be 和离它最近的主语的数保持一致。
Looking at the lunch, she ...
look 是she 主动发出的动作，因此用现在分词形式做状语【现在分词表示主动和进行】。
When she heard a noise from the bushes nearby, ...
这样的表达更好些。
... and hid herself behind ...
hide 的过去式是hid。
For a while, there was totally silence in the bushes. Mrs Stone thought the animal must have left. After which, she picked ...
after a while ... left 原句语法逻辑不通。hearing 是现在分词，它应该是句子主语【she】发出的动作，而后面的主语是the animal；must 后面跟动词原形。
She also telephoned the police, but they did not take it seriously.
the police，the 漏掉了；but 后面应该是主动语态，“警察没有认真对待这件事”。

祝学习愉快！