简单句只能说一件事，加上状语从句，句子一下子就有了逻辑：

If it rains tomorrow, we will stay at home. 条件

When he comes, I will tell him the news. 时间

I didn't go out because it rained. 原因

Although he is young, he knows a lot. 让步

状语从句 = 连词 + 一个完整的句子（有自己的主语和谓语）。从句放句首时，后面要加逗号；放句尾时不加。