As soon as the rain stopped, I called my friend and we all continued walking in the park.

as 在这里是连词，表示"当...的时候"，引导时间状语从句。 soon 是副词，意思是"不久、很快"，表示时间上的短暂。 as soon as 是一个固定搭配的连词短语，意思是"一...就..."，表示两个动作紧接着发生。