as long as和so long as均可表明“要是”，用以正确引导标准状语从句，二者没有什么差别。如：

I don’t care so [as] long as she lets me be with her son.要是她让我俩大儿子在一起，我无所谓。

You can take my car as [so] long as you drive carefully.你能我用的轿车，如果你当心一点儿开。

You may use my dictionary as [so] long as you don’t keep it too long.要是使用时间不过长，你能我用的字典。

可是，当as long as用以本意表明“与……一样长”时，与so long as在使用方法上是有差别的，即as long as可用以肯定句或肯定句，而so long as只用以毫无疑问，不用以肯定句。如：

It took three timesas long asI had expected.这件事情比我意料的多花了二倍的時间。

It didn’t take as [so] long as I expected.花的時间比我意料的短。