问几个语法知识点

1. 什么时候最高级前要加 the ,什么时候又不加啊?
这儿有2道题:
1) Gourds were introduced to what is now the southweatern US by earliest people who migreated north from M about 7000 years ago.
答案是 earliest -> the earliest

2) Although all sedimentary rocks contain iron, but the deposits that are richest in iron consist predominantly of minerals such as iron oxides, carbonates,...
很显然 but 应该去掉, 但是 richest前面却没有 the

2. 什么时候用 to do, 什么时候用 to doing
高中的语法好像讲过, 忘了.....

3. to do 能修饰名词吗? 在什么情况下可以修饰呢?

感激......