为什么状语从句变独立结构谓语要变成非谓语？

纠正一下，是含有状语从句的复合句变为简单句时，要将状语从句变成非谓语形式。即V—ing,V—ed，-to do形式。

1.分词作时间状语，常用在when， while，before，since，as引导的原因状语从句中。 He listened to music while walking.(= while he walked). 他边散步边听着音乐。

When completed(= when it is completed), the building will be painted white. 这幢楼在竣工后，将刷成白色。

2 .作原因状语，相当于because，since，as 引导的原因状语从句。 Not understanding this, he asked the teacher about.= Because he didn’t understand this,he asked the teacher about it. 因为不理解，关于那个问题他问了老师。

3 .作条件状语，相当于if， unless引导的条件状语从句。 Working hard, you’ll succeed. = If you work hard, you will succeed. 如果努力学习，你会成功的。

Given more time, he can finish the work. = If he is given more time, he can finish the work. 如果多给他点儿时间，他就能完成这项工作。

4 .作结果状语。 His parents died from the accident, leaving him alone in the world. 他的父母死于车祸，独留他于世。 【特别注意】过去分词一般不作结果状语。

5 .作方式或伴随状语。The teacher came in, followed by some students. 老师走了进来，身后跟着一些学生。

6 .作让步状语，相当于though， although， even if等引导的让不状语从句。 Lacking of money, he is happy every day= Though he lacks money, he is happy every day. 虽然缺钱，他每天还是开开心心地过日子。

7.作目的状语。To catch the early bus,he has to get up very early.=In order that he can catch the early bus,he has to get up very early.为了赶上早班车，他必须早早起床。

而独立主格结构是由两部分组成，前一部分是名词或者代词，后一部分是非谓语动词(不定式、现在分词、过去分词)或形容词、副词、名词或介词短语。前后两部分具有逻辑主谓关系。 非谓语动词作状语，其逻辑主语须与主句主语保持一致。若不一致，非谓语动词形式须另带主语，从而构成复合结构的形式作状语。这种结构称为“独立结构”。其中，非谓语动词主动用现在分词，被动用过去分词。

例如:Weather permitting, we will go there. 如果天气允许，我们就去。

His wallet stolen, he had to walk home. 钱包被盗了，他只得步行回家。

She lay there thinking, (with) her hands(crossing)behind her head. 她双手交叉放在头下，躺在那儿思考。

希望能帮到你！

把主语说两次，是不是很啰嗦？如果想把两个主语相同的句子合并，怎么办？非谓语动词就是应付这类啰嗦的利器之一。

在状语从句里，把从句主语省略掉，谓语变为非谓语即可。可以把这种变化理解为语言向简化方向的演变。

变化后，从句失去主语，借用主句的主语，谓语变为非谓语，成为状语短语修饰主句，从而和状语从句表达的意思一样。

此外，即使两个句子的主语不一样，也能变为独立主格。所以，状语从句转换为非谓语动词作状语，没啥大惊小怪。

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且看下面的例子：

不定式

• If you want to marry my daughter, you have to be excellent first.
• To marry my daughter, you have to be excellent first.
• 想要娶我女儿， 你首先要变得卓越。

现在分词

• When she heard she gained full marks in the test, she could not help smiling.
• Hearing she gained full marks in the test, she could not help smiling.
• 在听到自己在测试中得了满分后，她禁不住笑起来

过去分词

• If I was given more time, I can do it much better.
• Given more time, I can do it much better.
• 如果给更多时间，我能做得更好。

非谓语(成分)，顾名思义，就是不做谓语但可做主语、宾语、定语、状语、补足语的成分。

独立结构，说全了叫独立主格结构，是附加逻辑主语的非谓语形式。你给的例子，Being a boy, I fell in love with her.中的句首状语Being a boy 的逻辑主语就是句子的主语，因此不是独立结构。

替代状语从句的独立结构有二：①for/with+逻辑主格+不等式/动名词/过去分词，②介词+逻辑主语所有格+动作名词。以下举几个例子。

例1.The lady went into a dark room, followed with a maid.夫人走进一个阴森森的房间，后面跟着一女仆。说明：伴随状语也可写成：with a maid following.

例2.For/With the old here liking square dances, Bob is often upset.由于附近老人喜欢跳广场舞，鲍勃经常不得安宁。说明：句首状语的非谓语liking也可写成to like.

例3.For/With the products stolen in the mart, the firm set up several monitors.由于超市货物遭偷窃，店家在好几处安装了监视器。

例4.On my attention of his articles, you may attend as well.只要我关注他的文章，你不妨也跟着关注。说明：my是动作名词attention的逻辑主语，这样写更简洁。

例6.In teacher Liu's guiding, my son has got a great advance.由于刘老师的辅导，我儿子进步很大。说明：句首状语也可写成：With teacher Liu guiding.

小结：独立结构，逻辑严谨，简洁明快，几乎没有时态限制，可最大限度减少信息冗余，是英语入门的一个关键。记住这六个范例喔！

您举的这个例子不是独立主格结构，就是一般的非谓语动词。要回答你的问题，首先您要知道英语句子成分可以由词（大多为实词）来担任，可以由短语来担任，也可以由句子来担任。When I was a boy是句子作状语，一般的状语从句和定语从句都可以改成非谓语动词作状语和定语。非谓语动词作定语或状语与状语从句和定语从句的意思和语法功能是一样的，只是语法结构不同而已,一般都可相互转换。例如：When she found the course very difficult, she decided to move to a lower level.可以把状语从句改成非谓语动词作状语：Finding the course very difficult, she decided to move to a lower.又如：Lionel Messy,who has set the record for the most goals in a calendar year, is considered the most talented football player in Europe.可以把定语从句改成非谓语动词作定语：Lionel Messy,having set the record for the most goals in a calendar year, is considered the most talented football player in Europe.