be helpful in be helpful to be helpful for 三者区别即怎么用

1.be helpful in doing sth.
例如We hope this leaflet has been helpful in answering your questions.
我们希望这个宣传单可以帮助您找到您想要的答案
2.it is helpful (for sb) to do sth
It is helpful to discuss your problems with your friends.
-跟你的朋友们聊聊你遇到的问题是有好处的.这里的It是形式主语,真正的主语是to后面的句子
3.be helpful for sb对某人事有益的
例如;Music with the sounds of the ocean or bird song can be helpful for some.
大海的声音或者鸟叫声对你的睡眠时很有帮助的