By accident即不是及物动词也不是不及物动词，而是副词短语。

By accident中文意思是adv,偶然；意外地lt happened—whether by accident or design —that the two of them were left alone after all the others had gone.碰巧—不知是偶然还是有意安排—其他人走后，只剩下了他们两个人。