Understand的意思是理解懂得，强调的是状态，比如说我理解你现在不太高兴，I understand that you are not happy right now就是我明白现在是这种是不是这种状态。
而realize的意思是意识到，强调从本来不明白，本来不知道，本来没有知觉到变得知道理解的过程，所以经常的用法是come to realize that逐渐意识到。
realize是“认识到，意识到，实现”的意思。例如：
It's hard to realize one's own mistakes。
想要认识到自己的错误是困难的。
She flushed when she realized that she had made a foolish mistake。
当她意识到她犯了个愚蠢的错误，她脸红了。
He has realized his dream of going to college。
他实现了上大学生的梦想。
understand是指理解.什么意思之类的
如：I don't understand English.
我不懂英语.
realize 较understand 正式，realize的理解更深刻
realize:vt. 实现认识到了解将某物卖得，把(证券等)变成现钱变卖
I eventually realized my dream.
我终于实现了我的梦想。
understand:
vt. 理解懂获悉推断省略明白
vi. 理解懂得熟悉
The teacher talked a lot to the children， but they still didn't understand.
老师跟孩子们说了很多话，但他们还是不明白。
