only if用法：可修饰单数名词,也可修饰复数名词。前面常加定冠词the或形容词性物主代词。if only用法：用在名词短语之前,尤其是用在以数量词或数字起始的名词短语之前,起强调作用。

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Children are admitted only if accompanied by an adult.

儿童必须有成年人陪同方可入场。

Political attacks work only if they find an echo with voters.

政治攻击只有在选民中引起共鸣才会有作用。

If only I could have helped put matters right.

要是我当时能帮着把错误纠正过来就好了。

If only I knew her name.

我要是知道她的名字就好了。