1. think/ think/about/ think of
(1) think 单独使用时表示"思考"， 接that 宾语从句时意为"认为"，"觉得"。
I am thinking how to work out the problem. 
I think she is a good student.
当宾语从句含有否定概念时，通常形式上否定think ，但意义上却是否定宾语从句。
I don't think he can come.
I don't think it will be windy.
(2)think about 可接一个名词，动词-ing 形式或由疑问词引导的不定式或宾语从句，意思是"考虑……"。
I have thought about it for a long time.
Please think about how to tell her the bad news.
(3)think of 表示"认为"， 一般用于疑问句中，与what 连用。
    What do you think of the TV play? = How do you like the TV play?

2. big/ large/ great
上述形容词都表示"大"，但侧重点及程度不同。
  (1) big指具体事物的大小，强调比正常形体的标准大，既可用在普通场合， 也可用在正式场合。它可用来指人的身材高大或"长大了"，还可表示"伟大"，"重要"之意。如：
Can you lift up this big stone?
On the last day I made a big decision.
(2) large特别强调远远超过标准的大，指体积、面积、容积、数量之大。如：
A whale is a large animal.
A large crowd collected at the gate of the theatre.
(3) great除了表示数量体积之大外，又指抽象的程度，意味着伟大性，重要性， 优越性；常用于抽象
或无形的东西；用于有形的东西时，常带有"伟大"，"大得令人吃惊"等意思，含有一定的感情色彩。如：
China is a great country with a long history.
He was one of the greatest scientists.