我晕死,这种情况好像不只一种,比如说：
1.if only与only if
only if表示"只有"；if only则表示"如果……就好了".If only也可用于陈述语气.
e.g.
I wake up only if the alarm clock rings.只有闹钟响了,我才会醒.
If only the alarm clock had rung.当时闹钟响了,就好了.
If only he comes early.但愿他早点回来.
2.before long和long before
before long =soon,a little later不久以后,作时间状语,可用于将来时或过去时.
例：I hope to see you before long.
我希望不久能见到你.
Before long they had got everything ready.
不久他们就做好了一切准备.
long before=long long ago很久以前,表示过去,可单独作状语,其后还可接从句或名词.
例：We heard of it long before.
我们很久以前就听说过这件事.
It will be long before he arrives.
还要很久他才到达.
巧记：
long在前（long before）——“很久前”,
long在后（before long）——“不久后”.