only if 意思“僅僅如果”，用于引導(dǎo)條件狀語從句，比if所表達的語氣更強烈。置句首可引起倒裝。if only 用于虛擬語氣，表示“要是……就好了”。它用來表達一個不可能實現(xiàn)的愿望或假設(shè)。

與過去事實相反,后面跟過去完成時，had+過去分詞。

與現(xiàn)在事實相反,后面跟一般過去時。

與將來事實相反,后面跟一般過去時或should + 動詞原形。

例句辨析：

only if

1、I'll do that, but only if we set a few rules.

我可以干，但我們得定幾條規(guī)矩。

2、The placebo effect can be understood only if we acknowledge the unity of mind and body.

只有我們承認身心一體，安慰劑效果才能被理解。

3、The lawyer is paid only if he wins

只有官司贏了才支付律師費。

4、You can make progress only if you are modest.

惟虛心乃能進步。

5.You will win the race only if you train hard.

只要你努力訓(xùn)練，你會贏得比賽。

if only

1、If only it were that simple!

要是那么簡單就好了！

2、If only someone could help me!

有誰能幫助我就好了！

3、If only I could get some sleep, I would be able to cope.

要是能睡上一會兒，我就能應(yīng)付了。

4、If only I could be master of this subject.

我多么希望能精通這門學科。

5.if only I would driver the car.

要是我會開車就好了