cannot help do/doing和cannot help but do

cannot help do/doing和cannot help but do
1.cannot help but do
2.cannot but do
3.cannot help doing
4.cannot help do
请教以上几个短语的区别
I have no choice but to accept the fact
I can do nothing but accept the fact
上面这两句话为什么一个有to 一个没有？
老师说是固定搭配，这上面两句话是什么固定搭配？p50