一、基本用法的差别
1. as long as 有两个意思，一是用于本义，意为“与……一样长(久)”，在否定句中，也可说成 (not) so long as；二是用于引申义，表示“只要”，用于此义时也可说成 so long as(不管是肯定句、否定句还是疑问句)。如：
This river is as long as that one. 这条河与那条河一条长。
He lived abroad as long as five years. 他住在国外长达五年之久。
He didn't stay there as [so] long as she. 他在那儿住得不如她久。
As long as it doesn't rain we can go. 只要不下雨，我们就可以去。
As [So] long as you need me, I'll stay. 只要你需要我，我就留下。
2. only if 可视为 if 的加强说法，即其中的 only 是修饰 if 从句的，其意为“只有在……条件下”“……唯一的条件是…”。如：
I'll tell you, but only if you don't tell anyone else. 我可以告诉你，但条件是你不能告诉其他任何人。
I'll accept the bill only if you send it to my home address. 你必须把账单送到我住的地方，我才愿意偿付。
二、语气上的差别
从语气上看，only if 的语气要比as long as 强，as long as 通常只是用于提出一个条件，与if 的意思比较接近；而only if 引出的从句内容则通常暗示它是主句内容的唯一条件，含有“只有……才……”“只有在……的时候”“……唯一的条件是……”之意。比较：
As long as you do your best, we'll be happy. 只要你尽力，我们就满意了。
You can borrow the book as long as you keep it clean. 只要你不把书弄脏，这书你就可以借。
We can succeed only if everyone in the team pulls his weight. 我们全队只有人人各尽所能才能取得成功。
The coach stops at the principal hotels to pick up tourists, but only if they arrange this in advance. 大客车到各主要的旅馆来接观光者，但条件是必须事先安排好。
三、动词搭配上的差别
在许多情况下，as long as 与 only if 均可用，只是语气有强弱之分。但是，当主句和从句的谓语动词均为短暂性动词时，通常只用 only if，而不用 as long as。如：
你只有请他，他才会来。
误：He will come as long as you ask him.
正：He will come only if you ask him.
析：此句中的 only if 不能用 as long as 代替，因为动词 ask 与 come 均短暂性动词。
四、词序上的差别
两者均可引导从句置于主句之后，也可置于主句之前。但是，当only if 引导的状语从句置于主句之前时，其后的主句要用倒装语序，而且 as long as 引导的从句置于主句之前不用倒装。如：
As long as it doesn't rain we can play. 只要不下雨，我们就能玩。
As long as you're happy, it doesn't matter what you do. 只要你高兴，你做什么都没关系。
Only if a teacher has given permission is a student allowed to enter this room. 只有得到教师的允许，学生才可以进这间屋。
Only if the red light comes on is there any danger to employees. 只要红灯一亮，就表示有危及职工的险情。
五、注意 only if 与 if only 的差别
only if 意为“只有在……条件下”“唯一的条件是……”，可视为 if 的加强说法。
而 if only 有两个意思，一是表示“如果”“只要”，“也许”，“即使仅仅”，此时通常与陈述语气连用，与 only if 用法相似；二是表示“但愿”“要是……就好了”，此时多与虚拟语气连用。如：
If only they were here now, we would be able to celebrate their wedding anniversary.
如果他们现在在这里，我们就能庆祝他们的结婚纪念日了。
I will come only if you promise not to invite Mary. 只要你答应不请玛丽，我就来。
If only it clears up, we'll go. 只要天晴，我们就去。
He will succeed if only he does his best.
只要他全力以赴，他就会成功。
There had to be an answer — he was sure he could tease it out if only he had time.
一定会有答案——他肯定，只要他有时间就一定能弄个清楚。
She writes me often, if only to scold me because I haven't written to her.
她常来信，即使仅仅为了责怪我还没回信给她。
A one-to-one meeting with the US President was necessary, if only for a deeper exchange of views.
与美国总统一对一的会谈是必要的，即便仅仅是为了双方观点更深入的交流。
The pills might have helped him, if only he'd taken them regularly.
他当时要是按时服药，也许可以对他有帮助的。
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 you had told me that some time ago
要是你早些时候告诉我该多好啊！
If only he would hold her close to him.
要是他能将她拥入怀中就好了。
If only she could marry me!
她要是能嫁给我就好了。
If only I was rich. 要是我很有钱就好了。
If only I hadn't told him about it. 要是我没有告诉他这事就好了。
It's a good plan, if only we could carry it out. 那是个不错的计划，但愿我们能实现它。
If only he could conceal his part in the accident
要是他能够隐瞒住自己在这场事故中的责任就好了。
If only I could get some sleep, I would be able to cope.
要是能睡上一会儿，我就能应付了。
If only my father had possessed an ounce of business sense
要是我父亲有一点点经商的头脑就好了。
If only you hadn't felt compelled to meddle.
要是你没有觉得非插手不可就好了。
If only I could be master of this subject.
我多么希望能精通这门学科。
If only someone could help me!
有谁能帮助我就好了！
If only he arrived in time.
但愿他能及时赶到。
If only the West had been more responsive to his peace overtures in the fifties.
要是西方在50年代能更积极地响应他的和平姿态就好了。