1. 常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，这是为什么呢？譬如：
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》
2. 状语从句省略时，譬如 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？
常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什么呢？
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 doingare worth doing well.
【翻译】值得做的事情就值得做好。
这个What 没疑问的意思，它叫做“复合关系代词”（也叫“复合连接代词”），有两重用途：既是先行词又是关系代词。
关于what 复合关系代词的使用方法，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. Whats in the box is a big jar, and whats in the jar is a kind of wine.
从语法用途看，what 引导的这个句子叫做名词性从句，由于名词性从句有哪些用途就等于一个名词。那样，what 从句在主句中干什么成分呢？请看例句：
What we need is their help. （做主语）大家所需要的就是他们的帮忙。
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干什么成分呢？请看例句：
What appears difficult may be very easy. （做主语）看着非常难的事情或许比较容易。
What we liked most was his jokes.（做宾语）大家最喜欢的（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.
