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”. 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”.