偶觉得，要理解so much在这里的作用，是不是可以引入somwhat这样一个说辞来参考认识一下：
He is somewhat a farmer |
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He is not so much a farmer
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再比如：
He is so much a farmer （即，not so much的肯定式） |
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He is also a farmer
He is still a farmer |
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His nevertheless
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He is nevertheless a farmer
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由上面的比较例句可以看出，is 和表语之间的内容，本质上是修饰动词（这里是be动词第三人称单数现在时）的，说得宽泛点，是通过后置修饰be动词从而实现修饰be a farmer整个短语了。
即： be a farmer（是农民） be so much a farmer（真是个农民，太是个农民了） |
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be not somuch a farmer（并不怎么是农民，不怎么算是农民）
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He is from the very beginning a farmer
（不过，这句中的介宾结构修饰语通常选择后置于系表结构后） |
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于是，偶们来看主题的前半句：
His sin was not so much stealing the money 略去修饰语not so much，成为His sin was stealing the money（他的罪过是偷钱） 添加修饰成分： His sin was not so much stealing the money（他的罪过并不能怎么说是偷钱） |
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后半句，其实是和前半句各部相对应而对称的，即：
it was getting caught 因此，偶仍然觉得，这里it对应stealing the money逻辑上不通。 因为偶们省略前半句修饰语，那么是这样的： His sin was not stealing the money as it was getting caught |
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然后再来看as在这里的作用。首先偶们看一个比较例句：
He is a farmer as I am a teacher |
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从这个比较例句偶们知道，as在这里有轻微的转折意思，表示列举两种不同情况，可以理解成“而”或者“当…时”又或者“正如”等等，甚至通常可以不译出
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按照这样理解，偶们回看前面略去前半句中修饰语的那句话：
His sin was not stealing the money as it was getting caught （他的罪过不是偷钱，当它（——>这罪过）是被抓住时） |
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加上修饰语so much后：
His sin was not so much stealing the money as it was getting caught （他的罪过并不在于偷钱，而在于被抓（<——他的罪过不怎么能说是偷钱，当这罪过是被抓时）） |
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到此，解析完毕，偶的这次解析和数日前做的解析本质上（从生成语法在深层概念翻译层面上）是相同的。
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当然，这仍然是偶个人理解
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He was not so much a farmer at that time as he was a teacher
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