if only为何可以引导非真实条件句单独成句? 如：
Ifonly she would try again. 假如她能再试一次就好了。
Ifonly you had come to my birthday party. 你如果来了我的过生日晚会就好了。
假如ifonly是从属连词，由if only引导的用法虚拟语气的句子为何没主句？
ifonly...! 可以看作是一种惯使用方法。依据M.Swan的Practical EnglishUsage的说法，它表示一种愿望，比wish语气更强。Ifonly 引导的从句常常独立成句。如：
Ifonly I knew more people！
但也可以不独立成句。该书提供了两个有主句的例句：
1.If only it would sTOP raining, we would go out.
2.If only she hadn't told the police, everything would have been all right.
注意ifonly 单独成句时，句尾一般要用感叹号。不少词典都将ifonly归为习语，如 the freedictionary, Cambridge Dictionary 等。既然是习惯使用方法就不必深究为何可以没主句了。
