Understand的意思是理解懂得,強調的是狀態,比如說我理解你現在不太高興,I understand that you are not happy right now就是我明白現在是這種是不是這種狀態。
而realize的意思是意識到,強調從本來不明白,本來不知道,本來沒有知覺到變得知道理解的過程,所以經常的用法是come to realize that逐漸意識到。
realize是“認識到,意識到,實現”的意思。例如:
It's hard to realize one's own mistakes。
想要認識到自己的錯誤是困難的。
She flushed when she realized that she had made a foolish mistake。  
當她意識到她犯了個愚蠢的錯誤,她臉紅了。
He has realized his dream of going to college。
他實現了上大學生的夢想。
understand是指理解.什麼意思之類的
如:I don't understand English.
我不懂英語.
realize 較understand 正式,realize的理解更深刻
realize:vt. 實現認識到了解將某物賣得,把(證券等)變成現錢變賣
I eventually realized my dream.
我終於實現了我的夢想。
understand:
vt. 理解懂獲悉推斷省略明白
vi. 理解懂得熟悉
The teacher talked a lot to the children, but they still didn't understand.
老師跟孩子們說了很多話,但他們還是不明白。