这句话的意思和 He does not works so hard as he once did.并无不同.
但是 He works not so hard as he once did.这个句子中间还可以加几个字：
He (still) works,but not so hard as he once did.
not 的这种用法通常用在表达相反的类比的句子中,否定修饰动词的副词,如上例.试比较：
He learns English unhappily now.
He once learned English happily.
He learns English not so happily as he once did.
（He does not learn English as happily as he once did.注意前者是 so.as.,后者是 as.as.）
同一个语义可以用不同的方式表达.只要讲这个语言的人们认可,其表达方式就合乎语法.