I think it important to learn English well. 

请问这种句子是什么结构？ it 后面为什么不用is?

【答】这是复合宾语结构：think + 宾语 + adj. （形容词作宾补）。

你问“it 后面为什么不用is?” 其实，加is也可以，但结构变为：think + that ...（宾语从句）

I think （that）it is important to learn English well. 

相对而言，使用复合宾语结构，显得简练，因为它是个简单句。而加了is 后，成了复合句，结构变得复杂一些。

下面，我重点讲讲：英语的“6123结构”是什么意思？

在新课标人教版必修四第3单元中，有这样一个句子：You may find it astonishing that Charlie was taught to sing as soon as he could speak and dance as soon as he could walk. 句子意思是：“你可能会感到惊奇，在查理刚能说话时大人就教他唱歌，他刚能走路时大人就教他跳舞。”

本句含有一个常见的句型结构：find + it + adj. + that从句。句中的it作形式宾语，真正的宾语为that从句（其中that从句又包含一个由as soon as 引导的时间状语从句），astonishing 为形容词作宾语补足语。真正的宾语也可以由不定时或动名词充当。例如：

We found it quite pleasant to work with him. 我们发现跟他一块工作很愉快。

I felt it hard to communicate with them. 我感觉难以跟他们交流。

He considers it quite foolish to do like that. 他认为那样做很愚蠢。

We think it no use complaining. 我们认为抱怨是没有用的。

该句式中的it 作形式宾语。为了记忆方便，我们称该句型为“6123结构”。

【说明】这种称谓仅仅是为了学习和记忆，而且，这些动词是英语学习中最常见的，不一定包揽所有情况。

“6”指主句中6个常用动词：think, consider, believe, make, find, feel。“1”指形式宾语it ；“2”指宾补的两种形式：形容词或名词；“3”指真正宾语的三种形式：①不定式短语，②动名词短语，③that引导的宾语从句。

This special strain of rice makes it possible to produce one-third more of the crop in the same fields. 这种特殊的稻种能够在同样的田里增加三分之一的农作物产量。（必修四第2单元）

This produced a chain reaction, which made it possible for life to develop. 这产生了连锁反应，使得生命孕育发展成为可能。（必修三第4单元）

He didn’t make it clear when and where the meeting would be held. 我们还没弄清楚会议将在何时何地召开。

She made it a rule to get up at six and take a walk in the morning. 她在早晨六点起床然后散步已成为习惯。

Do you find it funny to see someone sliding on a banana skin? 看见有人踩在香蕉皮上滑倒，你觉得有趣吗？

The teacher feels it his duty to help the student with his English. 老师感到，帮助学生学习英语是自己的责任。

We thought it necessary that the two sides should stop arguing. 我们认为，双方停止争论是必要的。

I consider it rude to stare at other people. 我认为，眼睛盯着别人看是无礼的。

We believe it useless reading without understanding. 我们认为，不理解的情况下阅读是没用的。

He considered it a good thing that the parliaments would be involved. 他认为，议会将卷入其中是好事一桩。

Ann felt it an justice that she had been automatically blamed. 安觉得，她已自动接受职责是公正的。

They find it a mystery that he has never mentioned his parents. 他从来没提到过自己的父母，他们发觉真是个谜团。

We consider it necessary that the machine should be oiled regularly. 我们认为，有必要对这台机器经常加油润滑。

Under the circumstances I feel it right that you should know. 在这种情况下，我觉得你应该知道。

I found it strange that she had not come yet. 她竟然没来，我发现奇怪。

Mr. Black made it clear that the students should prepare every lesson. 布莱克先生明确指出，学生应准备每一节课。

I think it important that we should keep calm. 我认为我们保持冷静是很重要的。

I find it difficult to do the job well. 我发现做好这件事不容易。

I consider it best that you should stay here. 我认为你最好住这儿。

I find it easy to get on with her. 我发现她很容易相处。

They haven’t made it known where they are to hold the conference. 他们还没宣布会议在哪里开。

【特别说明】上述6个动词中，除了make 以外，其他5个动词think, consider, believe, find, feel，都可以接两种结构：

1. think：

think + 宾语 + 宾补（形容词或名词充当）

think + that...

2. consider

consider + 宾语 + 宾补（形容词或名词充当）

consider + that...

3. believe

believe + 宾语 + 宾补（形容词或名词充当）

believe + that...

4. find

find + 宾语 + 宾补（形容词或名词充当）

find + that...

5. feel

feel + 宾语 + 宾补（形容词或名词充当）

feel + that...