By day, Robert Titterton is a lawyer. In his spare time though he goes on stage beside pianist Maria Raspopova—not as a musician but as her page turner. “I’m not a trained musician, but I’ve learnt to read music so I can help Mariain in her performance.”

上面一段摘自2021年新高考英语全国卷I的阅读理解B。句中有两组 not…but… 句式。对于第一个 not…but… 比较好理解：

not as a musician but as her page turner 不是作为音乐家，而是作为翻乐谱的人

但对于第二个 not…but… 就不好理解：

I’m not a trained musician, but I’ve learnt to read music so I can help Mariain in her performance.

因为这里的 not…but…连接的不是相同的句子结构：not 后面是名词短语，but 后面是一个句子。