Otherwise a child could feel hopeless and helpless to do anything about it.

VOA: special.2010.05.06

And now, you're not the driver, you really feel helpless until you notice standing next to you, leaning over the bridge is a very fat man.

但你不是电车司机，你真的爱莫能助,直到你发现，在你旁边，靠着桥站着的,是个超级大胖子。

耶鲁公开课 - 公正课程节选

You use "I can't help but" when you feel helpless, when you want to get something across that needs to happen.

当你觉得无助时，或者当你说明需要做到这件事情的时候，你就可以使用“I can't help but”句型。

I can't help but 课堂 - SpeakingMax英语口语达人

And so you feel helpless until you notice that there is, off to the right, a side track and at the end of that track, there is one worker working on the track.

你极为无助,直到你发现，在轨道的右侧，有一条侧轨,而在侧轨的尽头，只有一名工人在那施工。

耶鲁公开课 - 公正课程节选