1）若but前没有do，则不定式要带to。
a. I have no choice but to wait .
b. He needs nothing but to succeed.
2）若but前有do，but后的不定式省略to。
a. I have nothing to do but wait.
b. He will do anything but give in. 他决不会屈服。
3）在解释do的精确含义时，be动词后直接加动词原形，省略to。
a. All that I could do then was wait .
b. What I could do then was wait .
c. All you should do now is complete this form.
d. No mountain is too high for you to climb. All you have to do is have some faith. No river is too wide for you to make it across. All you have to do is believe it when you pray.
e. —I have a very demanding job. I’m always tired when I get home.
— I also have a demanding job, but I’m expected to come home from the office and cook, care for the children and clean the house. All you do is sit and watch television and complain about how tired you are.