He looked helpless sitting there all alone. 他一个人坐在那儿，显得很无助。
be/feel helpless to do sth I knew they were lying but I felt helpless to do anything about it. 我知道他们在撒谎，可我对此却无可奈何。
This witticism reduced both of them to helpless laughter. 这句俏皮话让他俩不由自主地大笑起来。