Almost immediately he began to complain about the weather, for even though it was still summer, it rained continually and it was often bitterly cold.目前自己了解even though it was still summer是让步状语从句，it rained continually and it was often bitterly cold.是这个让步状语的主句。但Almost immediately he began to complain about the weather,算什么，算整个句子的主句 然后for 引导缘由状语从句？完全不同这句子的层次。

网友忽略了这是一个并列句。第二并列分句由 for （由于）引导。

Almost immediately he began to complain about the weather, ——第一并列分句。

for even though it was still summer, it rained continually and it was often bitterly cold.—— 第二并列分句。

在第二并列分句中，又包括一个主从句，从句由 even though 引导。