but可作连词、介词、副词用：

连词：She is young but very experienced

介词：Nobody knew her but me.

We had no choice but to wait.

He did nothing all day long but watch TV.

副词（相当于 only ） Tom is but a child：

记住but作介词表示除……之外时，后跟名词、代词、不定式（前面句子无do）或动词原形（前面句子有do），只要会用就行了，切勿死扣语法，钻牛角尖，因为语法无法解释语言万象。