unless = if...not.

e.g. Let's go out for a walk unless you are too tired.

If you are not too tired, let's go out for a walk.

if 引导的条件句有真实条件句和非真实条件句两种:

真实条件句，

如：If it rains tomorrow, we won’t go on a picnic.

非真实条件句是虚拟语气的一种，表示与事实相反，如：

If I were you, I would go with him.

so/as long as只要

由as (so) long as, in case引导。

So long as you’re happy, it doesn’t matter what you do.

只要你高兴，你做什么都没有关系。

You may borrow my book as long as you keep it clean.

只要你保持书的清洁，你就可以把我的书借去。

Take your umbrella in case it rains.

带着你的伞吧，以防下雨。