somebody cannot but do something 是正式短语，通常用于书面，不用于口语，意为“不得不”。

多种表达“不得不…；非…不可；不禁…”的短语如下，cannot可缩写为can't：

can't help being so cute

书面：
cannot but do
cannot choose but do

- I cannot but admire her. 我不得不佩服她。
- I cannot choose but go at once. 我不得不马上去。

口语：
cannot help but do
cannot help doing【注意cannot help后面的两种不同动词形式】
cannot help oneself from doing
have no (other) choice/alternative but to do【注意有to】

- I cannot help laughing. 我不禁笑起来。
- I had no choice but to resign. 我不得不辞职。