The second you think it's so late (主语)is (系动词)the fastest moment.(表语) The second (先行词)you think it's so late (定语从句) you(主语) think (谓语)it's so late (宾语从句) it's so late (主系表)
The second (when) you think it's so late is the fastest moment. 整个句子为复合句, (when) you think it's so late 为从句-------定语从句,The second is the fastest moment 为主句。 关系副词 when ,where ,why 在定语从句中可以省略(口语) 时间有限,我先回答这些。