经常有人会说主语从句是引导词加完整句子而定语从句是加不完整句子这是为何呢？

What is worth doing is worth doing well.这个主语从句就是啊《从句的主语what代替了啊》

—— What is worth doing is worth doing well.

= All that is worth doing is worth doing well.

= Everything that is worth doing is worth doing well.

= All things that are worth doing are worth doing well.

【翻译】值得做的事情就值得做好。

这个What 没有疑问的意思，它叫做“复合关系代词”（也叫“复合连接代词”），有两重作用：既是先行词又是关系代词。

关于what 复合关系代词的用法，下面再举几例，请试着翻译一下：　　

1. The old man lived in what was called a small forest.　　

2. What surprised me most was his good memory.　　

3. What's in the box is a big jar, and what's in the jar is a kind of wine.　　

从语法作用看，what 引导的这个句子叫做名词性从句，因为名词性从句的作用就相当于一个名词。那么，what 从句在主句中做什么成分呢？请看例句：　　

What we need is their help. （做主语）我们所需要的就是他们的帮助。　　

He has got what he wants. （做宾语）他得到了他想要的（东西）。　　

That is what I heard on the bus.（做表语）那就是我在公共车上听到的故事。　　

what 本身在它引导的从句中做什么成分呢？请看例句：　　

What appears difficult may be very easy. （做主语）看起来很难的事情也许很容易。　　

What we liked most was his jokes.（做宾语）我们最喜欢的（东西）就是他讲的笑话。　China is not what she was more than 50 years ago.（做表语）中国已不是50多年前的中国了。　　

状语从句的省略时Write to me if necessary. 说是从句省略了it is难道还原主句 还能是it

write to me么？  ——是if 从句省略了it is，不是主句。补充完整就是：Write to me if it is necessary.