从句之前都有个成分引导
we will go for a walk if there is no rain tomorrow.
像这句,主句是we will go for a walk
从句是there is no rain tomorrow,它由if引导
而且if是“如果”的意思,表假设,所以这是条件状语从句.
如果你完全没了解过,还是看语法书较具体