①本句是一个主从复合句。

②it marks the beginning of the hottest season ...是主句；as在此句中引导________状语从句，意为"因为，由于"。

【拓展】as的其他用法如下：

★引导让步状语从句，意为"尽管"。如：

　Child as he is, he knows a lot.

　Hard as he tried, he didn't pass the exam.

★引导方式状语从句，意为"以......的方式"。如：

　She loves singing just as her mother does.

★引导时间状语从句，意为"当......时"。如：

　As he did his homework, he looked through the Web.

【句意】由于它（端午节）是在初夏，它标志着一年中最炎热的季节的开始。

【仿写】由于春节是在冬末，它标志着一年中最寒冷的季节的结束。

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答案

1.【分析】It is thought that they have discussed the problem.

　It is said that they are cleaning their classroom at present.

【仿写】Nowadays, football is said to be the most popular sport in the U.K.

或Nowadays, it is said that football is the most popular sport in the U.K.

2. 【分析】原因

【仿写】As the Spring Festival is in late winter, it marks the end of the coldest season of the year.