葛老在《英语惯用法词典》(p.334)中说：“注意下面第一句里用‘I know it’，第二句用‘I know’：
He is going to get married, and I know it.
When his sister said that he was going to get married, I said, 'Yes, I know.'
但是，我不明白为什么第一句子用I know it，而在第二个句子里用I know。
根据现代英语语法学家的解释，在英国英语中用I know，而在美国英语中用I know it。
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I asked when she was leaving, and she said she didn't know. 我问她什么时候走，她说她不知道。(know 后面的成分未出现，可用省略来解释。)
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His father came here yesterday. —Yes, I know. (Palmer) 他父亲昨天来了。——是的，我知道。
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I reconstructed the crime and before I had finished speaking, Boyle said, 'A tissue of lies!' I asked the accused if he really thought so and he answered, 'Think so? I know it!' (LEG) 我描述了那桩罪行，博伊尔没等我说完就喊道：“连篇的谎言！”我问被告是否真的这么想，他回答说：“这么想？我知道那是谎言！”
[附注]有时候,I know it 中的 it 并不是指前句话的内容,而是指它所提到的某个事物。这时候,it 当然是不可省略的。例如:
‘I went to a nice restaurant called The Elizabeth last night.’ ‘I know it.’ “昨天晚上我到一家挺好的饭馆去了，名叫伊丽莎白。”“我知道这家饭馆。”