as long as 和 so long as 均可表示“只要”，用於引導條件狀語從句，兩者沒什麼區別。如：

I don’t care so [as] long as she lets me be with her son. 只要她讓我和她兒子在一起，我不在乎。

You can take my car as [so] long as you drive carefully. 你可以用我的汽車，只要你小心點兒開。

You may use my dictionary as [so] long as you don’t keep it too long. 只要使用時間不太長，你可以用我的詞典。

但是，當 as long as 用於本義表示“與……一樣長”時，與 so long as 在用法上是有區別的，即 as long as 可用於肯定句或否定句，而 so long as 只用於否定句，不用於肯定句。如：

It took three times as long as I had expected. 這件事比我預料的多花了兩倍的時間。

It didn’t take as [so] long as I expected. 花的時間比我預料的短。