①You shouldn't give a child more money than needed.
②We will do works as planed.
③I don't have such a pen as needed.
各位专家老师们，我想了解的是as和than加过去分词的使用方法是限于as和than分句的哪一类型型，譬如在现代语法as和than划分为了比较从句，到了传统语法as和than被划分到了定语从句，同时as又分为了非限定语从句和限制性定语从句。
但在as和than+过去分词的使用方法中呢，假如as根据传统语法划分为限制性定语从句则不能用直接加ed分词的使用方法，因此句③错误，假如as作非限定语从句则可以。（记得原来在哪篇文章看过，不确定）
同时还有as加形容词的使用方法，这种使用方法是不是把as看成了介词，省略了being？
烦请各位老师不吝赐教，谢谢!
所以这几个句子用现代语法比较分句讲解最好。
①You shouldn't give a child more money than needed.
= You shouldn't give a child more money than is needed.
= You shouldn't give a child more money than it is needed to give him.
it为形式主语，不定式为外置主语（真的主语），形式主语和外置主语可以同时省略。然后is可以进一步省略。than needed为比较级more的补足语。
②We will do works as planned.
= We will do works as is planned.
= We will do works as it is planned that we should do.
it为形式主语，that从句为外置主语。形式主语和外置主语可以同时省略，得到as is planned。is可以进一步省略，得到as planned。
as planned 为比较分句作方法状语
③I don't have such a pen as needed.
= I don't have such a pen as is needed.
such a pen为比较项，其对应性在比较分句做主语是需要省略的。is可以进一步省略。as needed为比较分句作such的补足语。