as long as 和 so long as 均可表示“只要”，用于引导条件状语从句，两者没什么区别。如：
I don't care so [as] long as she lets me be with her son. 只要她让我和她儿子在一起，我不在乎。
You can take my car as [so] long as you drive carefully. 你可以用我的汽车，只要你小心点儿开。
You may use my dictionary as [so] long as you don't keep it too long. 只要使用时间不太长，你可以用我的词典。
但是，当 as long as 用于本义表示“与……一样长”时，与 so long as 在用法上是有区别的，即 as long as 可用于肯定句或否定句，而 so long as 只用于否定句，不用于肯定句。如：
It took three times as long as I had expected. 这件事比我预料的多花了两倍的时间。
It didn't take as [so] long as I expected. 花的时间比我预料的短。
