I recieved a letter,saying that a girl would come.这叫做什么状语从句啊？

这叫做什么状语从句啊？
句子是不是相当于I recieved a letter and it said that a girl would come.
是不是？

这叫伴随状语从句。一般由现在分词+从句的结构组成伴随状语，表示动作与主句同时发生。
原句可以相当于I recieved a letter and it said that a girl would come.

这是一种从句 一一＂ saying his brother was in hospital ＂是用来修饰＂a letter＂的 本来是两个句子：l just got a letter． a letter says his brother was in hospital．两句合并成一句时改成ing形式就可以了～

这是动词ing形式做伴随状语，是收到的那封letter的伴随的状态，意思是（信上说....）。 对，可以理解成I recieved a letter and it said that a girl would come.或者I recieved a letter which said that a girl would come.