"This exam was really difficult." "I know. I struggled with every question."

I 是第一人称代词，指代说话者自己。 know 是动词，意思是"知道、了解"。 I know 是一个常用表达，表示说话者认同对方的说法或感受，相当于汉语的"我知道"或"我明白"。