When my best friend invited me to her party, I did not think twice and said yes immediately.

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"或"再次"。 not think twice 是一个固定搭配，字面意思是"不考虑两次"，引申为"毫不犹豫"或"不假思索"的意思。