When she saw the elderly woman struggling with her bags, she did not think twice and went over to help.

not 是否定词，表示"不"或"没有"。 think 是动词，意思是"思考、考虑"。 twice 是副词，意思是"两次"，这里与 think 搭配使用。 not think twice 是一个固定短语，字面意思是"不考虑两次"，引申为"毫不犹豫"的意思。