Almost immediately he began to complain about the weather ,for even though it was still summer ,it rained continually and it was often bitterlly cold .
句意是：几乎很快地,他开始抱怨天气,因为即使那时候是在夏天,（由于）经常不停地下雨,天气也会有点冷.
1）整个长句是一个从句（也叫复合句,由多个简单句组成）,从句有主句部分和从句部分
主句是Almost immediately he began to complain about the weather
for even though it was still summer ,it rained continually and it was often bitterlly cold
是从句部分,且是原因状语从句
for的搭配常常是for+原因,所以for常常引导原因状语从句
2 ) 在for even though it was still summer ,it rained continually and it was often bitterlly cold
这个原因.状语从句中又包含了一个让步状语从句“even though it was still summer .”
（even though ,even if ,though 这一类表示 假设、转折的词语常常接状语从句的）
注：这个长长的句子是挺复杂的,它是从句中包含了从句
关于从句的语法是高中和大学语法的重点,建议你多学习一下关于从句的语法,考试经常考的.