在than引导的比较从句中what的去留问题，是英语学习者的一个难点。
《牛津用法指南》第256词条有这样一个例句：
He worries more than (what) is necessary.
该书作者 Michel Swan 指出 than 后的 what 必须去掉。作者认为than 类似关系代词，可以代替其后的主语或宾语，该书原话为："than" and "as" can take the place of subjects in clauses(rather like relative pronouns)。但是，在英语书刊中我们可以经常看到很多than后使用what的例句，如：
1. In fact, compared to hundreds of millions of people around the world, everyone here has more than what is necessary.
2. This means that the consequences of the measure should not extend further than what is necessary to combat the avoidance.
3. A narration will be brief if it does not ask more than what is necessary for the case or ramble on further than is necessary.
我想这些句子可能给一些网友带来困惑，难道这些句子都是错误的?显然不是。我认为《牛津用法指南》的作者讲解的不够全面，没有进一步分析什么情况下what 必去，什么情况下必须保留以及什么情况下what可有可无。下面我浅谈一下自己对这一用法的体会。
我们先比较一下两个例句：
1. He worries more than is necessary. （《牛津用法指南》的例句）
2. In fact, compared to hundreds of millions of people around the world, everyone here has more than what is necessary. (From Every Angle, 2000, p.207).
在句1中，worries 为不及物动词, more than is necessary 可以理解为：more than it is necessary that he should worry. 按照Swan的解释，than 类似关系代词，或称准关系代词, 替代了后面的it. 但是，这种解释也不是很全面，因为按照现代英语的用法，很多情况下than 后的be动词也可以去掉。
请看外刊例句：
1. Do you think you worry more than necessary?
2. The patient agrees that he tends to worry a little more than necessary about minor matters.
我认为句1之所以不能有what, 一个重要的原因是worry是不及物动词，more than (is) necessary.这个短语实际上起副词的作用，修饰worry的程度。如果用what is necessary, 主句中不仅没有和what相对应的比较对象，而且变成了名词性结构，无法发挥副词的作用。类似例句如：
1.As programmers we do not want to have to worry more than is necessary about this process.
2. Also, a person who sees forgetfulness as a sign that they are “losing their marbles” may worry more than is necessary.
3. Personal data shall not be kept longer than necessary.
但句2 则不同，has 是及物动词，more than what is necessary实际上起到了has的宾语的含义，即这里的每个人都拥有的超过了实际需要。因此使用名词结构what is necessary=the thing that is necessary. 作为主句has的宾语比较对象, 所以此分句可以使用what。请看相关例句：
1. Promotions go to the one who is willing to do more than what is necessary to get by.
2. The depreciation of implements is not more than what is necessary.
3. If a Christian will do no more than what is necessary, he will quickly be tempted to omit something of that also.
当than从句比较对象为主句主语时（一般为非指人的主语），what也可以保留。如：
1. In these circumstances，no more cattle can be fed in the stable than (what) are necessary for tillage .
2. Is the solution faster or cheaper than (what) is already available?
3. That innovation is suggested by a series of poems in which typological efficacy extends further than (what) is considered typological, even by those who stretched Reformed definitions.
根据现代英语的用法，以上例句中如果去掉than后的what is也是正确的。请注意，有些情形可以省略what, 但不能省略be 动词，《剑桥语法》提供了部分可以这样使用的词。此外，请看以下补充例句：
1. Although one should not assume more than necessary, one should also not assume less than necessary to model a phenomenon.
2. If he has more than is necessary for his actual expenditures, it would not be the most conducive to his training or his education.
3. When one person had more than necessary, he or she was effectively hoarding the excess at the expense of the others.
关于what的这种用法，WR论坛母语网友也有过解释：What in this context is also often used to replace that which. It sounds less formal and is used especially in speech. The what can sometimes be omitted, but again, it may make it sound more formal:
"This contains far more ingredients than (what/that which) was described in the menu."
"This is far more difficult than what/that which I was told it would be."
https://forum.wordreference.com/threads/than-what.204707/
下面我们分析一下 what 必须去掉、必须保留和可有可无的三种情形：
