Can't always understand when people talked to me.求解WHEN的作用及这

Can't always understand when people talked to me.求解WHEN的作用及这是什么用法?
It wasn't easy for me to understand the teacher when she talked.
Can't always understand when people talked to me.
不要翻译,请给教以下WHEN 的用法.不只限此句.关与when的都行.
thanks!
就是问下 when在这些句子是什么成分呢？
这两句是否是 从句？