Is there any rule for when to use to and when to drop it in expressions that include but (to) do?
A1-- You have no choice but to study hard.
B1-- You have no choice but study hard.
A2 -- I can't help but keep asking questions.
B2 -- I can't help but to keep asking questions.
A3 -- She could do nothing but give in.
B3 -- She could do nothing but to give in.
I guess all A's are correct. What about the B's? If they are incorrect, is it because of a rule, some logic, or because of idiomatic usage?