句子1：She had learned to swim when she was 5 years old. （错误）

         She learned to swim when she was 5 years old.（正确）

         【注意】learn 后面作宾语的动词只能用不定式，不能用动名词。

句子2：I am glad to see you.

         【注意】在 be glad/happy 这种结构后面只能用动词不定式（不能用动名词）作原因状语。