As long as, so long as, only if, on (the) condition that, providing (that) 和 provided (that) 都是連接詞，用來引導對某一情況施加特定條件或設定限制的條件句。它們可謂同義詞，意為「只要」，但用法不盡相同。As long as 和 providing (that) 比較常用於口說，而 so long as, only if, on (the) condition that 和 provided (that) 則比較正式且比較常用於書寫。例如：
- Tom can come as long as he can leave by 5.00. (只要湯姆能在五點鐘之前出發，他就能來)
- So long as you don’t drive too fast, you can borrow the car. (只要你別開太快，你就可以借這部車)
- I’ll come to the party only if you don’t wear those ridiculous trousers! (我會參加聚會，只要你不要穿那條丟人現眼的褲子！)
- I can lend you 10,000 dollars on (the) condition that you can repay the money within three months. (我可以借你一萬元，只要你能在三個月內償還這筆錢)
- I will go providing (that) you go too. (只要你也去，我就去)
- Provided (that) there is no opposition, we shall hold the year-end banquet here. (只要無人反對，我們就在這裡舉辦尾牙)
必須注意的是，雖然這些條件句大多指未來，但由於它們屬於副詞子句，所以要使用現在簡單式來表示未來。例如 (以 as long as 為例)：
- I’ll always remember you as long as I live. (只要我活著，我會永遠記著你) (正)
I’ll always remember you as long as I will live. (誤) - Simon is welcome to come along as long as he behaves himself. (只要賽蒙守規矩就歡迎他來) (正)
Simon is welcome to come along as long as he will behave himself. (誤)