what's the difference?

其實你用得多，你就會知道有咩分別的了。
agree to 中文為同意，多數都是用在一個人的身上。
agree to sb
E.g. I agree to Max about his opinion.
agree on 多數都是用在一樣東西或一項 term 上面的。
agree on sth.
E.g. I agree on Max's opinion.
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know 是解作「知道」
E.g I know that you are a careful person.
know of 是指「知道某人或某事物的情況」
E.g. Do you know of anyway to stop a person snoring ?

Yogi的答案不太正確

agree on 意思是 "雙方 (對某一問題) 達成一致意見
Both sides agreed on a price for the house.
雙方同意這座房子的價格。
通常用於formal d 既野，如兩國達成一致意見

agree with 意思是 "同 (某人) 的意見一致，適合，符合"
agree with sth 係可以
e.g. Ladies love shopping. I totally agree with it. (女士們愛購物，我對此十分認同)