想请教一下老师们，as表示“作为”含义时，在句子中充当的是什么成分？位置有什么要求？具体疑惑如下：

1. As an important language, English is learned by many students in China.

2. English as an important language is learned by many students in China.

3. English is learned by many students in China as an important language.

这三个句子正确的是哪个或者哪几个？这里as引导的是什么状语？

另外，想问一下for也有“作为”的含义吗？

如：She was chosen as/for our class monitor.

The house was built as/for a library.

可是为什么词典查不到for有“作为”的意思？