if only的用法

If only 表示虚拟语气，如
If only I were you!如果我是你.

If only (expressing a strong wish表示强烈的愿望)但愿，真希望，只要...就好了：
If only he wouldn't eat so noisily.但愿他吃东西不这样大声就好了。/If he would only learn to eat quietly!但愿他学会吃东西时小声点就好了！

If only+称述句 "要是...就好了/该有多好啊!"
该句用虚拟语气,其构成如下:
1. 对已发生的动作的感叹,从句谓语用过去完成时
If only I hadn't lied to her yesterday!
2. 对现在的动作的感叹,从句谓语用一般过去时(be动词用were)
If only I were a millionaire!
3. 对将来的动作的感叹,从句谓语用过去将来时
If only he would come here tomorrow!

if only
adv.
1. 要是 (表示虚拟语气,动词的时态要用过去时,be动词一律用were. 如果动作本身就发生在过去,那就要用过去完成时.)
If only it hadn't!
要是没有登就好了!
If only I could swim.
要是我会游泳该多好.
If only I were rich.
但愿我很富.
If only she would come!
如果她能来就好了！
If only I had seen it.
要是我看见就好了。
If only someone could help me!
有谁能帮助我就好了！
2. (表示强烈的愿望) 但愿，真希望，只要...就好了：

If I only knew!
要是我知道该多好。
If only he will!
他愿意就好了！
Oh, if only he will!
要是他愿意就好了！