●过去式 过去分词是thought thought
It’s the thought that counts.
Different people have different thoughts.
●I think 作插入语,位置灵活，可置于句首句尾句中，用逗号隔开。
I think,Tom has known the truth. Tom,I think, has known the truth.
Tom has known the truth, I think.
●do you think作插入语，后接宾语从句，疑问词放在句首，句中没有逗号，句式结构是：
疑问词+do you think+陈述语序的句子…？
What do you think made him so angry? 你认为是什么事让他那么生气？ Who do you think he is running after? 你认为他在追赶谁？
I thinkTom has known the truth.
I think后没有标点符号，它不是插入语，是主句，Tom has known the truth.是宾语从句。
●否定前移及反意问句 I think后跟宾语从句，变否定句时，否定词需要前移，即把主句I think变成否定句，不能把宾语从句变否定句。
I don’t think Tom has known the truth.我想汤姆不知道真相。
● it 作think的形式宾语，真正的宾语是后面的动词不定式，句式解构为：
主+think+it+adj+(for sb)+to do
I think it helpful to learn a foreign language.
I think (that) it’s helpful to learn a foreign language.
I think him very clever.
=I think (that) he is very clever.
●I think so.
I don’t think so.
① think of 想到，记得，认为,of是介词。 She always thinks of others 。
I can’t think of her name.
② think about 思考，考虑，认为，about是介词。
He is thinking about a math problem. 他在思考一道数学难题。
※当表示“认为，对某人某物有什么看法”时，think of和think about可以互换。 What do you think of/about the film? 你认为这部电影怎么样？
③ think over 仔细考虑，over是副词。 宾语是代词时，要放在think over中间。
Think over, and you will find a good way.仔细想想，你就会找到好办法的。 Let me think it over.让我仔细想想。
④ think twice( about) 再三思考，慎重考虑，和think over意思接近。
You must think twice before you make any big decision.
Think twice about it and you will have a good idea.慎重考虑一下，你就会有好注意的。
⑤ think up =come up with 想出,提出(主意，计划等）
We have thought up a good idea.
⑥ think hard 仔细想，费力地琢磨，深思
They are thinking hard about their holiday plans.他们正在费劲地琢磨他们的假期计划。
其实，还是think about 的用法，只不过多了一个副词hard。
⑦ think back to 回想起，回忆起
I often think back to those times. 我经常回想起那些时候。