Question

can't ... but后面什么时候加do什么时候加to do?

麻烦给个归纳性的说话，然后加几个例子，别只给例子,只说用法。回答得好的有加分。

Answer 1

but是转折连词，正规的书面语当中，but左右要对称
can 后面要接动词原形
你说的can't...but并不是固定用法，完全可以分开说的。也就是说，并不像one...another;so...that;too...to这种常用句式……
所以需要分开说，如果but后面接了to do,but前面必然要有to do出现
如果某些情况下非要把can't加在前面了，那can't后面必将紧接动词原形出现
比如
it can't be right to say but to practice其中to say和to practice对称出现。be为动词原形
i can't tell you but i can still give you some hint其中can't tell和can give对称出现。tell和give为动词原形

Answer 2

(1) cannot/can't help + ing (FORMAL)
(2) cannot/can't help but + based form of the verb
I can't help but correct your mistakes.
I can't help teaching if you don't like it.