一.only if是由充目前置修飾語的副詞only和從屬連詞if組合而成，only僅僅起突出作用，并不改變if的詞義（only if有時會分開，表示唯一的條件）：隻有、除非,意思相當于unless,當用于肯定句,引導條件狀語從句：
The lawyer is paid only if he wins.
隻有官司赢了才支付律師費。
Syria will attend only if the negotiations deal with issues of substance.
隻有在談判涉及重大問題時叙利亞才會參加。
Children are admitted only if accompanied by an adult.
兒童必須有成年人陪同方可入場。
I will go only if you go with me.
除非你跟我一起走,否則我就不走（隻有你跟我一起走我才走）.
Political attacks work only if they find an echo with voters.
政治攻擊隻有在選民中引起共鳴才會有作用。
The meat market can continue only if it is radically overhauled to conform with strict European standards.
肉類市場隻有經過徹底的大檢查，嚴格遵守歐洲标準，才能繼續營業。
注意：Only if 用于句首，用倒裝語序:
Only if you pass the entrance exam and become a college student, will I accept you as a friend.
隻要你通過聯考，成為大學生，我就會認你為朋友。
Only if the red light comes on is there any danger to employees.
隻有紅燈閃亮時才有危及職工的險情。
Only if we persist in carrying out the open door policy, will we achieve greater success in every field.
隻要我們堅持改革開放，我們就能在各個方面取得更大的成就。
Only if a teacher has given permission is a student allowed to enter this room.
隻有得到老師的允許，學生才能進入這間教室。
二.if only 僅僅是if的強化形式，不僅用在虛拟語氣中，用以表達願望或非真實條件；也可以用在真實條件句的情形。
用在虛拟語氣是可以引導單獨句子；也可以引導從句和主句一起使用。
1.當if only用在虛拟語氣是可以引導單獨句子用法如下：
（1）表達過去已發生的事件是不能或未曾實作的願望，常含有後悔、惋惜的意思，此時謂語動詞要用過去完成時：
If only you had told me that some time ago
要是你早些時候告訴我該多好啊！
If only you hadn't felt compelled to meddle.
要是你沒有覺得非插手不可就好了。
If only the West had been more responsive to his peace overtures in the fifties.
要是西方在50年代能更積極地響應他的和平姿态就好了。
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。
Oh! If only I'd known earlier.
嗳，早知道就好了。
Look at the trouble I am in! If only I had followed your advice.
看我現在所處的困難，當初要采納了你的建議就好了。（上海聯考）
特例：用could have done來表達過去已發生的事件是不能或未曾實作的願望的虛拟語氣：
If only I could have helped put matters right.
要是我當時能幫着把錯誤糾正過來就好了。牛津高階英漢雙解詞典
（2）表達與現在事實相反的假設，即表示遺憾、惋惜等。謂語動詞通常用虛拟語氣過去時：
If only Marx were still by my side to see this with his own eyes!
馬克思今天如能同我站在一起親眼看到這種情景，那該多好啊!
If only I were rich.
我要是富有該多好啊！
If only I knew her name.
我要是知道她的名字就好了。
If only it were that simple!
要是那麼簡單就好了！
If only I could swim!
我要是會遊泳就好了！
（3）表示與将來的事實可能相反的假設，謂語動詞使用would/could+do;或虛拟語氣過去時，例：
If only he would hold her close to him.
要是他能将她擁入懷中就好了。
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。
If only he could conceal his part in the accident
要是他能夠隐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.
我多麼希望能精通這門學科。
If only she could marry me!
她要是能嫁給我就好了。
If only someone could help me!
有誰能幫助我就好了！
If only it would clear up soon!
但願天氣趕快放晴。
If only he arrived in time.
但願他能及時趕到。
If only the banks would lend money without limit!
但願銀行能無限制地貸款！
2.當if only引導從句和主句一起使用虛拟語氣；分以下情況：
（1）. 表示遺憾：當我們對過去的某種情況感到遺憾時，可以使用"if only"來表達。例如：
If only I had studied harder, I would have passed the exam.
如果我當初學習更努力一些，我就會通過考試了。
If only I hadn't missed the train, I would have been there on time.
如果我沒有錯過火車，我就會準時到達那裡了。
（2）. 表示願望：當我們對現在的某種情況感到不滿或對未來有某種期望時，可以使用"if only"來表達。例如：
If only I had more time, I could finish this project.
如果我現在有更多的時間，我就能完成這個項目了。
If only it would stop raining, we could go for a walk.
如果雨能停下來，我們就可以去散步了。
（3）. 表示非現實情況：當我們對某種不太可能發生的情況抱有幻想時，可以使用"if only"來表達。例如：
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。
If only I were taller, I could play basketball professionally.
如果我再高一些，我就可以成為職業籃球運動員了。
If only I had a million dollars, I would travel around the world.
如果我有一百萬美金，我就會環遊世界。
（4）. 與過去完成時連用：當使用"if only"表示對過去的遺憾時，通常會與過去完成時連用。例如：
If only I had known it, I wouldn't have troubled him.
要是我知道，我就不會麻煩他了。
If only I had told her the truth, everything would be different.
如果我當時告訴了她真相，一切都會不同。
If only I hadn't lost my phone, I wouldn't be in such a mess now.
如果我沒有丢失手機，我現在就不會陷入這樣的困境了。
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。
If only Mr. Le had told me yesterday, I would have given him some help, but now it is too late.
如果李先生昨天告訴我，我将會給他一些幫助。但是，現在為時已晚。
（5）. 與過去簡單時連用：當使用"if only"表示對現在的遺憾或對未來的願望時，通常會與過去簡單時連用。例如：
If only I had more energy, I would exercise every day.
如果我現在有更多的精力，我就會每天鍛煉身體。
If only I had more money, I could buy some new clothes.
要是我有更多的錢，我可以買些新衣服。
If only I had a car, I could drive to work instead of taking the bus.
如果我有一輛車，我就可以開車上班，而不用坐公共汽車了。
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。
（6）. 與過去進行時連用：當使用"if only"表示對過去的某種情況進行假設時，可以與過去完成進行時連用。例如：
If only I were studying at that time, I would have understood the concept better.
如果我當時在學習，我就能更好地了解這個概念了。
If only I had been paying attention, I wouldn't have missed the turning.
如果我當時注意觀察，就不會錯過轉彎了。
3.當if only引導從句和主句一起使用，不使用虛拟語氣；也能引導（真實）條件狀語從句：
If only he gets this job, it will make a great deal of difference.
他要是能得到這個工作，情況就會大不相同。
If only we can get to the next petrol-station we’ll be all right.
要是我們能達到下一個加油站，我們就好辦了。
If only I had known earlier，I'd have sent you a telegram.
我隻要知道得早一點，就會拍電報給你了。
I’m sure he is up to the job if only he would give his mind to it.
隻要他肯下定決心，我相信他能勝任這份工作。
If only it clears up, we'll go.
隻要天一放晴,我們就走。
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，隻要他有時間就一定能弄個清楚。
If only she would listen to what I was saying we might be able to sort out this problem.
隻要是她願意聽我說，我們也許就能夠解決這個問題。
If only our horse can get ahead of the leading runner, it can win the race!
隻要我們的馬超過領先的賽手，它便能赢得勝利了！