1. Only if is a combination of the adverb only and the subordinate conjunction if, which acts as a preposition modifier, only only plays a prominent role and does not change the meaning of the word if (only if is sometimes separated, indicating the only condition): only, unless, the meaning is equivalent to unless, when used in affirmative sentences, leading conditional adverbial clauses:
The lawyer is paid only if he wins.
Attorney fees are paid only if the lawsuit is won.
Syria will attend only if the negotiations deal with issues of substance.
Syria will participate only if the negotiations involve significant issues.
Children are admitted only if accompanied by an adult.
Children must be accompanied by an adult to enter.
I will go only if you go with me.
I won't go unless you go with me (I won't go until you go with me).
Political attacks work only if they find an echo with voters.
Political attacks will only work if they resonate with voters.
The meat market can continue only if it is radically overhauled to conform with strict European standards.
The meat market can only continue to operate if it has undergone a thorough inspection and strict compliance with European standards.
Note: Only if is used at the beginning of the sentence, in inverted order:
Only if you pass the entrance exam and become a college student, will I accept you as a friend.
As long as you pass the college entrance examination and become a college student, I will recognize you as a friend.
Only if the red light comes on is there any danger to employees.
Only when the red light is flashing can there be a danger to the employees.
Only if we persist in carrying out the open door policy, will we achieve greater success in every field.
As long as we persist in reform and opening up, we will be able to achieve greater achievements in all aspects.
Only if a teacher has given permission is a student allowed to enter this room.
Students are only allowed to enter this classroom with the permission of the teacher.
2. If only is only an enhanced form of if, not only in the subjunctive mood to express wishes or non-real conditions;
Used in the subjunctive mood to lead a single sentence, or to lead a clause and a main clause together.
1. When if only is used in the subjunctive mood it can lead to separate sentence usage as follows:
(1) Expressing that events that have occurred in the past are wishes that cannot or have not been fulfilled, often containing regret and regret, at this time the predicate verb should be in the past perfect tense:
If only you had told me that some time ago
If only you had told me earlier!
If only you hadn't felt compelled to meddle.
If only you didn't feel like you had to get involved.
If only the West had been more responsive to his peace overtures in the fifties.
If only the West had responded more positively to his peaceful gestures in the 50s.
If only my father had possessed an ounce of business sense
If only my father had a little bit of a business mind.
Oh! If only I'd known earlier.
Hey, if only I knew.
Look at the trouble I am in! If only I had followed your advice.
Looking at the difficulties I find myself in, I wish I had taken your advice in the first place. (Shanghai College Entrance Examination)
Special case: The subjunctive mood that could have done to express that events that have happened in the past are wishes that cannot or have not been fulfilled:
If only I could have helped put matters right.
If only I could have helped correct the mistake at the time. Oxford Advanced English-Chinese Dictionary
(2) Expressing assumptions that are contrary to the current facts, i.e., expressing regret, regret, etc. Predicate verbs are usually in the subjunctive mood past tense:
If only Marx were still by my side to see this with his own eyes!
How nice it would be if Marx could stand with me today and see this with my own eyes!
If only I were rich.
How nice it would be if I were rich!
If only I knew her name.
If only I knew her name.
If only it were that simple!
If only it were that simple!
If only I could swim!
If only I could swim!
(3) to express the assumption that it may be contrary to the facts in the future, and the predicate verb uses would/could+do; or subjunctive mood in the past tense, for example:
If only he would hold her close to him.
If only he could hold her in his arms.
If only I could get some sleep, I would be able to cope.
If I can sleep for a while, I'll be able to handle it.
If only he could conceal his part in the accident
If only he could hide his responsibility for this accident.
If only I could be master of this subject.
How I wish I could be proficient in this subject.
If only she could marry me!
If only she could marry me.
If only someone could help me!
If only someone could help me!
If only it would clear up soon!
Let's hope the weather clears up soon.
If only he arrived in time.
Hopefully he'll be there in time.
If only the banks would lend money without limit!
Hopefully, banks can lend without restrictions!
2. Use the subjunctive mood when if only leads the clause and the main clause together;
（1）. Expressing regret: When we regret a certain situation in the past, we can use "if only" to express it. For example:
If only I had studied harder, I would have passed the exam.
If I had studied harder, I would have passed the exam.
If only I hadn't missed the train, I would have been there on time.
If I hadn't missed the train, I would have been there on time.
（2）. Expressing wishes: When we are unhappy with a certain situation in the present or have some expectations for the future, we can use "if only" to express it. For example:
If only I had more time, I could finish this project.
If I had more time now, I would have been able to complete this project.
If only it would stop raining, we could go for a walk.
If the rain could stop, we could go for a walk.
（3）. Indicate a non-reality situation: When we have fantasies about something that is unlikely to happen, we can use "if only" to express it. For example:
If only I could get some sleep, I would be able to cope.
If I can sleep for a while, I'll be able to handle it.
If only I were taller, I could play basketball professionally.
If I had been taller, I would have become a professional basketball player.
If only I had a million dollars, I would travel around the world.
If I had a million dollars, I would travel the world.
（4）. Conjunctive with Past Perfect Tense: When "if only" is used to indicate regret about the past, it is often used conjunctively with the past Perfect tense. For example:
If only I had known it, I wouldn't have troubled him.
If I knew, I wouldn't have bothered him.
If only I had told her the truth, everything would be different.
If I had told her the truth at the time, everything would have been different.
If only I hadn't lost my phone, I wouldn't be in such a mess now.
If I hadn't lost my phone, I wouldn't be in such a predicament now.
The pills might have helped him, if only he'd taken them regularly.
If he had taken his medication on time, it might have helped him.
If only Mr. Le had told me yesterday, I would have given him some help, but now it is too late.
If Mr. Lee had told me yesterday, I would have given him some help. However, it is too late now.
（5）. Conjunction with Past Simple Tense: When "if only" is used to express regret for the present or wishes for the future, it is often used in conjunction with the past Simple tense. For example:
If only I had more energy, I would exercise every day.
If I had more energy now, I would exercise every day.
If only I had more money, I could buy some new clothes.
If I had more money, I could buy some new clothes.
If only I had a car, I could drive to work instead of taking the bus.
If I had a car, I could drive to work instead of taking the bus.
If only I could get some sleep, I would be able to cope.
If I can sleep for a while, I'll be able to handle it.
（6）. Conjunctive with Past Continuous Tense: When "if only" is used to make assumptions about a situation in the past, it can be used conjunctively with the past perfect continuous tense. For example:
If only I were studying at that time, I would have understood the concept better.
If I had been studying, I would have understood the concept better.
If only I had been paying attention, I wouldn't have missed the turning.
If I had been watching, I wouldn't have missed a turn.
3. When the if only presiding clause is used together with the main clause, without using the subjunctive mood, it can also lead the (real) conditional adverbial clause:
If only he gets this job, it will make a great deal of difference.
If he had gotten the job, it would have been very different.
If only we can get to the next petrol-station we’ll be all right.
If we can get to the next gas station, we'll be fine.
If only I had known earlier，I'd have sent you a telegram.
As long as I knew earlier, I would have sent you a telegram.
I’m sure he is up to the job if only he would give his mind to it.
As long as he is willing to put up his mind, I believe he can do the job.
If only it clears up, we'll go.
As soon as the sky clears, we will go.
There had to be an answer — he was sure he could tease it out if only he had time.
There will be an answer – he's sure he'll figure it out if he has time.
If only she would listen to what I was saying we might be able to sort out this problem.
As long as she's willing to listen to me, we might be able to solve this problem.
If only our horse can get ahead of the leading runner, it can win the race!
As soon as our horse overtakes the leading fancier, he will win!