下列句子结构是什么?另外,介词短语算是句子结构吗?如是,是什么语?

She received a letter from her brother last week.主谓宾状.she是主语received 谓语a letter 宾语from her brother 状语last week.状语
There are a lot of people at the bus stop.这是倒装句.即there are是谓语.a lot of是定语,people是主语,at the bus stop是地点状语
I borrowed a book from the library this morning.主谓宾状.I 主语borrowed谓语 a book宾语 from the library this morning状语
He left immediately.主谓状.He 主语left 谓语immediately状语.
注：介词短语可做各种成份,定语,状语,表语等等.

完全同意 小幽ice 的回答。from her brother, at the bus stop, from the library都是介词短语，在句中都可理解地点状语（具体的方位或抽象的来源等）。
另，介词短语在句中的语法功能多种多样，可具体分析。