if当是否讲时引导什么从句

从句可放在主句前面，此时要用逗号分开，也可放在主句后面。

如：

it doesn't rain tomorrow , we will go shopping .（if it doesn't rain tomorrow是从句）

= We will go shopping if it doesn't rain tomorrow.

如果明天不下雨，我们就去购物。

will come if you invite him.   (if you invite him 是从句）

=If you invite him , he will come .

如果你请他,他会来的。

if引导的从句称之为条件状语从句，在句子中作状语。主句用将来时态，而从句用一般现在时。