一、if only可表示對現時或未來的愿望,也可表示與過去事實相反的愿望,作“只要…但愿,要是…就好了”解。
第一種用法：相當于as long as 只要，連接2個句子，從句是條件狀語從句：
There had to be an answer — he was sure he could tease it out if only he had time.
一定會有答案——他肯定，只要他有時間就一定能弄個清楚。
If only it clears up,we’ll go.只要天一放晴，我們就去。
第二種用法：要是……就好了；如果……就好了；但愿
用來表達一個不可能實現的愿望或假設，句子多用虛擬語氣，偶爾有時也用陳述語氣：If only she would come!要是她能來就好了。
If only she come early.要是她早點來就好了。
If only I knew then what I know now.
要是當初我就懂了我到現在才領悟的東西，該多好啊！
其虛擬語氣用法如下：
（一）表示與現在事實相反的假設，用一般過去時：
If only she could marry me! 她要是能嫁給我就好了。
If only I were as clever as you.要是我像你一樣聰明該有多好??！
If only I could have helped put matters right.
要是我當時能幫著把錯誤糾正過來就好了。
If only I knew her name.
我要是知道她的名字就好了。
If only I were rich.
但愿我很富有。
If only I could get some sleep, I would be able to cope.
要是能睡上一會兒，我就能應付了。
《柯林斯高階英漢雙解學習詞典》
She writes me often, if only to scold me because I haven’t written to her 她常來信，即使僅僅為了責怪我還沒回信給她。
If only it were that simple!
要是那么簡單就好了！
If only he could conceal his part in the accident
要是他能夠隱瞞住自己在這場事故中的責任就好了。
If only I could be master of this subject.
我多么希望能精通這門學科。
If only someone could help me!
有誰能幫助我就好了！
If only he arrived in time.
但愿他能及時趕到。
Oh, if only he knew English!
唉，要是他懂英語該多好??！
（二）表示與過去事實相反的假設，用過去完成時：
If only we had arrived in time,we would,we would not have missed the train. 如果我們及時到達，我們就不會錯過火車了。
If only he had been here.要是他當時在這兒就好了。
The pills might have helped him, if only he’d taken them regularly.
他當時要是按時服藥，也許可以對他有幫助的。
《牛津高階英漢雙解詞典》
If only you had told me that some time ago
要是你早些時候告訴我該多好??！
If only somebody had told us, we could have warned you.
如果有人告訴了我們,我們就能事先提醒你。
If only I had another chance.要是我再有一次機會就好了。
Look at the trouble I am in! If only I had followed your advice.
看看我現在的困境，要是我當時采納你的建議就好了。
If only my father had possessed an ounce of business sense
要是我父親有一點點經商的頭腦就好了。
If only you hadn’t felt compelled to meddle.
要是你沒有覺得非插手不可就好了。
Oh! If only I’d known earlier.
噯，早知道就好了。
摘自《漢英大詞典》
（三）表示與將來事實相反的假設，用would +動詞原形
If only it would stop raining.但愿雨快停下來。
If only he would hold her close to him.
要是他能將她擁入懷中就好了。
If only the banks would lend money without limit!
但愿銀行能無限制地貸款！
摘自《柯林斯高階英漢雙解學習詞典》
If only it would clear up soon!
但愿天氣趕快放晴。
二、only if 表示：唯一的條件是，只有在……的條件下（的時候）才
放在句首，句子倒裝：
Only if the red light comes on is there any danger to employees.
只有紅燈閃亮時才有危及職工的險情?！杜＝蚋唠A英漢雙解詞典》
Children are admitted only if accompanied by an adult.
兒童必須有成年人陪同方可入場。
Syria will attend only if the negotiations deal with issues of substance.
只有在談判涉及重大問題時敘利亞才會參加。
The lawyer is paid only if he wins
只有官司贏了才支付律師費。
《柯林斯高階英漢雙解學習詞典》
Use a fire extinguisher only if you have been trained to do so ．
只有經過訓練后才使用滅火器。高考真題- 2018 天津
So, everyone can have a good fortune only if he tries his best.
所以，每個人只有拼盡全力才能獲得好運。
