When she came into the living room,her mother was reading a letter from her father.

When she came into the living room,是从句,
her mother was reading a letter from her father.是主句.
由关联词when, while, as, because, until, before, after etc.等这些词引导的句子,通常是从句,剩下的那个句子就是主句.这样的句子叫状语从句